Information reproducing apparatus and method, and computer program

ABSTRACT

An information reproducing apparatus ( 1 ) is provided with: an offset adding device ( 19 - 1, 19 - 2 ) for adding a first offset value (OFS) which can be set to be variable, to a read signal (R RF ) read from a recording medium ( 100 ); a correcting device ( 18 ) for correcting waveform distortion occurring in a read signal corresponding to a long mark, of the read signal to which the first offset value is added; an offset subtracting device ( 19 - 2, 19 - 3 ) for subtracting a second offset value (OFS) which can be set to be variable, from the read signal in which the waveform distortion is corrected; and a waveform equalizing device ( 15 ) for performing a waveform equalization process on the read signal in which the second offset value is subtracted.

Technical Field

The present invention relates to an information reproducing apparatusand method which reproduce record data recorded on a recording medium,and particularly relates to an information reproducing apparatus andmethod which perform waveform equalization, such as a filtering process,on a read signal obtained by reading the record data recorded on therecording medium, as well as a computer program which makes a computerfunction as the information reproducing apparatus.

BACKGROUND ART

In order to improve an SN ratio of a read signal read from the recordingmedium on which the data is recorded at high density, there is known atechnology by which a filtering process for emphasizing high frequenciesis performed on the read signal, for waveform equalization. Inparticular, according to a patent document 1, it discloses thetechnology by which the high frequencies can be emphasized without anyintersymbol interference by performing the filtering process afteramplitude limit is performed on the read signal (a technology about aso-called limit equalizer).

-   Patent document 1: Japanese Patent No. 3459563

DISCLOSURE OF INVENTION Subject to be Solved by the Invention

Here, waveform distortion can occur in the read signal. The waveformdistortion indicates such a status that there is a discrepancy between aproper signal level to be taken and a signal level that actually appearsin the read signal. If the waveform distortion is included in a range inwhich the amplitude limit is performed on a limit equalizer (i.e. ascoherency between the waveform distortion and an amplitude limit valueon the limit equalizer increases), the waveform distortion is furtheremphasized by high-frequency emphasis performed after the amplitudelimit. For example, this likely leads to a disadvantage that a mark witha relatively long run length is misjudged to be another mark.Specifically, for example, this likely leads to a disadvantage that amark with a run length of 8 T is misjudged to be a mark with a runlength of 4 T, a space with a run length of 2 T, and a mark with a runlength of 2 T.

The disadvantage of misjudging the mark is not limited to be on thelimit equalizer but may be on various waveform equalizers, such as aPRML (Partial Response Maximum Likelihood) system.

In view of the aforementioned conventional problems, it is therefore anobject of the present invention to provide an information reproducingapparatus and method which can preferably reproduce the record data evenif the waveform distortion occurs, as well as a computer program.

Means for Solving the Subject

The above object of the present invention can be achieved by aninformation reproducing apparatus provided with: an offset adding devicefor adding a first offset value which can be set to be variable, to aread signal read from a recording medium; a correcting device forcorrecting waveform distortion occurring in a read signal correspondingto at least a long mark, of the read signal to which the first offsetvalue is added by the offset adding device; an offset subtracting devicefor subtracting a second offset value which can be set to be variable,from the read signal in which the waveform distortion is corrected; anda waveform equalizing device for performing a waveform equalizationprocess on the read signal in which the second offset value issubtracted.

The above object of the present invention can be also achieved by aninformation reproducing method provided with: an offset adding processof adding a first offset value which can be set to be variable, to aread signal read from a recording medium; a correcting process ofcorrecting waveform distortion occurring in a read signal correspondingto at least a long mark, of the read signal to which the first offsetvalue is added by the offset adding process; an offset subtractingprocess of subtracting a second offset value which can be set to bevariable, from the read signal in which the waveform distortion iscorrected; and a waveform equalizing process of performing a waveformequalization process on the read signal in which the second offset valueis subtracted.

The above object of the present invention can be also achieved by acomputer program for reproduction control and for controlling a computerprovided in an information reproducing apparatus provided with: anoffset adding device for adding a first offset value which can be set tobe variable, to a read signal read from a recording medium; a correctingdevice for correcting waveform distortion occurring in a read signalcorresponding to at least a long mark, of the read signal to which thefirst offset value is added by the offset adding device; an offsetsubtracting device for subtracting a second offset value which can beset to be variable, from the read signal in which the waveformdistortion is corrected; and a waveform equalizing device for performinga waveform equalization process on the read signal in which the secondoffset value is subtracted, the computer program making the computerfunction as at least one portion of the offset adding device, thecorrecting device, the offset subtracting device, and the waveformequalizing device.

The operation and other advantages of the present invention will becomemore apparent from the embodiments described below.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram conceptually showing the basic structure of aninformation reproducing apparatus in an example.

FIG. 2 is a block diagram conceptually showing the structure of a limitequalizer in the example.

FIG. 3 is a waveform chart conceptually showing an operation of settingthe upper limit and the lower limit of an amplitude limit value, on asample value series.

FIG. 4 are waveform charts conceptually showing an operation ofobtaining a high-frequency emphasized read sample value series, on thesample value series.

FIG. 5 are waveform charts conceptually showing a first example ofwaveform distortion.

FIG. 6 are waveform charts conceptually showing a second example ofwaveform distortion.

FIG. 7 is a flowchart conceptually showing a flow of operations of anadder, an offset addition circuit, a subtracter, and a waveformdistortion correction circuit.

FIG. 8 is a block diagram conceptually showing the structure of thewaveform distortion correction circuit.

FIG. 9 is a waveform chart conceptually showing an operation ofcorrecting the waveform distortion by the waveform distortion correctioncircuit, on the sample value series.

FIG. 10 is a waveform chart conceptually showing a waveform or the likeof a read signal before and after the correction of the waveformdistortion.

FIG. 11 are waveform charts conceptually showing the operation ofobtaining the high-frequency emphasized read sample value series, on thesample value series, in each of a case where the waveform distortion isnot corrected and a case where the waveform distortion is corrected.

FIG. 12 is a graph showing a change in symbol error rate with respect toa waveform distortion ratio.

FIG. 13 are graphs showing a change in symbol error rate with respect toan offset value normalized by the amplitude of the read signal, in eachof a case where the offset value is only added (i.e. where the offsetvalue is not subtracted) and a case where the offset value is added andsubtracted.

FIG. 14 is a waveform chart conceptually showing a waveform of the readsignal corresponding to minT according to a change in asymmetry.

FIG. 15 is a waveform chart conceptually showing another waveform or thelike of the read signal before and after the correction of the waveformdistortion.

FIG. 16 is a waveform chart conceptually showing an asymmetry value.

FIG. 17 are tables showing the appearance probability of the record datawith each run length.

FIG. 18 is a waveform chart conceptually showing a β value.

FIG. 19 is a waveform chart conceptually showing a partial β value.

FIG. 20 is a waveform chart conceptually showing an α value.

FIG. 21 is a flowchart conceptually showing another flow of operationsof the adder, the offset addition circuit, the subtracter, and thewaveform distortion correction circuit.

FIG. 22 is a flowchart conceptually showing another flow of operationsof the adder, the offset addition circuit, the subtracter, and thewaveform distortion correction circuit.

FIG. 23 is a waveform chart conceptually showing an operation ofcorrecting the waveform distortion by a waveform distortion correctioncircuit provided for an information reproducing apparatus in a firstmodified example, on the sample value series.

FIG. 24 is a block diagram conceptually showing the structure of thewaveform distortion correction circuit provided for the informationreproducing apparatus in the first modified example.

FIG. 25 is a waveform chart conceptually showing an operation ofcorrecting the waveform distortion by a waveform distortion correctioncircuit provided for an information reproducing apparatus in a secondmodified example, on the sample value series.

FIG. 26 is a block diagram conceptually showing the structure of thewaveform distortion correction circuit provided for the informationreproducing apparatus in the second modified example.

FIG. 27 is a waveform chart conceptually showing an operation ofcorrecting the waveform distortion by a waveform distortion correctioncircuit provided for an information reproducing apparatus in a thirdmodified example, on the sample value series.

FIG. 28 is a block diagram conceptually showing the structure of thewaveform distortion correction circuit provided for the informationreproducing apparatus in the third modified example.

FIG. 29 is a waveform chart conceptually showing an operation ofcorrecting the waveform distortion by a waveform distortion correctioncircuit provided for an information reproducing apparatus in a fourthmodified example, on the sample value series.

FIG. 30 is a block diagram conceptually showing the structure of thewaveform distortion correction circuit provided for the informationreproducing apparatus in the fourth modified example.

FIG. 31 is a timing chart conceptually showing an operation ofcorrecting the waveform distortion by a waveform distortion correctioncircuit provided for an information reproducing apparatus in a fifthmodified example, on a first read signal.

FIG. 32 is a timing chart conceptually showing the operation ofcorrecting the waveform distortion by the waveform distortion correctioncircuit provided for the information reproducing apparatus in the fifthmodified example, on a second read signal.

FIG. 33 is a flowchart conceptually showing a first flow of operationsby the waveform distortion correction circuit provided for theinformation reproducing apparatus in the fifth modified example.

FIG. 34 is a flowchart conceptually showing a second flow of operationsby the waveform distortion correction circuit provided for theinformation reproducing apparatus in the fifth modified example.

FIG. 35 is a block diagram conceptually showing the structure of awaveform distortion correction circuit provided for an informationreproducing apparatus in a sixth modified example.

FIG. 36 is a block diagram conceptually showing the structure of awaveform distortion detection circuit provided for the waveformdistortion correction circuit provided for the information reproducingapparatus in the sixth modified example.

FIG. 37 is a plan view schematically showing marks on a recordingsurface of a read-only type optical disc.

DESCRIPTION OF REFERENCE CODES

-   1, 2 information reproducing apparatus-   10 spindle motor-   11 pickup-   12 HPF-   13 A/D converter-   14 pre-equalizer-   15 limit equalizer-   16 binary circuit-   17 decoding circuit-   18 waveform distortion correction circuit-   181 delay adjustment circuit-   182 distortion-correction-value detection circuit-   183 mark/space length detection circuit-   184 timing generation circuit-   185 selector-   186 waveform distortion detection circuit-   19-1 adder-   19-2 offset generation circuit-   151 amplitude limit value setting block-   1516 averaging circuit-   152 amplitude limit block-   1522 interpolation filter-   1523 limiter-   153 high-frequency emphasis block

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, as the best mode for carrying out the present invention, anexplanation will be given on embodiments of the information reproducingapparatus and method, and the computer program of the present invention.

(Embodiment of Information Reproducing Apparatus)

An embodiment of the information reproducing apparatus of the presentinvention is an information reproducing apparatus provided with: anoffset adding device for adding a first offset value which can be set tobe variable, to a read signal read from a recording medium; a correctingdevice for correcting waveform distortion occurring in a read signalcorresponding to at least a long mark, of the read signal to which thefirst offset value is added by the offset adding device; an offsetsubtracting device for subtracting a second offset value which can beset to be variable, from the read signal in which the waveformdistortion is corrected; and a waveform equalizing device for performinga waveform equalization process on the read signal in which the secondoffset value is subtracted.

According to the embodiment of the information reproducing apparatus ofthe present invention, for example, by the operation of the offsetadding device, the first offset value is added to the read signal. Thefirst offset value can be set to be variable, and the offset value canbe changed as occasion demands. At this time, the addition of the firstoffset value may be performed once per the read signal, or a pluralityof times per the read signal in a stepwise manner.

Then, by the operation of the correcting device, the waveform distortionis corrected which occurs in the read signal corresponding to at leastthe long mark (e.g. marks with run lengths of 7 T to 11 T and 14 T ifthe recording medium is a DVD, and marks with run lengths of 6 T to 9 Tif the recording medium is a Blu-ray Disc). Here, the waveformdistortion (and more specifically, for example, the signal level or thelike of the waveform distortion) is preferably corrected such that thewaveform distortion does not have an adverse effect on the waveformequalization by the waveform equalizing device (and specifically, forexample, on amplitude limit and high-frequency emphasis filteringdescribed later).

Then, by the operation of the offset subtracting device, the secondoffset value is subtracted from the read signal in which the waveformdistortion is corrected. The second offset value can be set to bevariable, and the second offset value can be changed as occasiondemands. At this time, the subtraction of the second offset value may beperformed once per the read signal, or a plurality of times per the readsignal in a stepwise manner.

Then, by the operation of the waveform equalizing device, the waveformequalization process is performed on the read signal in which the secondoffset value is subtracted. Then, various signal processes (e.g. abinary process, a decoding process, and the like) are performed on thewaveform-equalized read signal, and thus, the record data is reproduced.

As described above, since the waveform distortion of the read signal iscorrected after the first offset value is added to the read signal, evenif relatively large asymmetry occurs in the read signal, it is possibleto preferably prevent such a disadvantage that the signal level of aspace that constitutes the record data with a relatively short runlength, which is assumed to be originally greater than or equal to areference level, is less than or equal to the reference level (or zerolevel, and the same shall apply hereinafter). If the signal level of thespace that constitutes the record data with a relatively short runlength is less than or equal to the reference level, the record data islikely misjudged to be the waveform distortion. However, even if theoccurrence of the asymmetry causes the signal level of the space thatconstitutes the record data with a relatively short run length, which isassumed to be originally greater than or equal to a reference level, tobe less than or equal to the reference level, the addition of the firstoffset value to the read signal allows the signal level of the space tobe greater than or equal to the reference level. Namely, it is possibleto preferably prevent such a disadvantage that the record data with arelatively short run length is not misjudged to be the waveformdistortion. Incidentally, here, it is aimed at the recording medium inwhich reflectance is reduced by recording the record data (in otherwords, in which the reflectance of the mark is less than that of thespace).

In the same manner, in the recording medium in which the reflectance isincreased by recording the record data (in other words, in which thereflectance of the mark is greater than that of the space), even if therelatively large asymmetry occurs in the read signal, it is possible topreferably prevent such a disadvantage that the signal level of thespace that constitutes the record data with a relatively short runlength, which is assumed to be originally less than or equal to areference level, is greater than or equal to the reference level (orzero level, and the same shall apply hereinafter). If the signal levelof the space that constitutes the record data with a relatively shortrun length is greater than or equal to the reference level, the recorddata is likely misjudged to be the waveform distortion. However, even ifthe occurrence of the asymmetry causes the signal level of the spacethat constitutes the record data with a relatively short run length,which is assumed to be originally less than or equal to the referencelevel, to be greater than or equal to the reference level, the additionof the first offset value to the read signal allows the signal level ofthe space to be less than or equal to the reference level. In otherwords, it is possible to preferably prevent such a disadvantage that therecord data with a relatively short run length is misjudged to be thewaveform distortion.

Moreover, since the second offset value is subtracted after the waveformdistortion is corrected, it is possible to relatively reduce a load forthe generation or calculation of the first offset value and the secondoffset value, as detailed later. In other words, it is possible togenerate or calculate the first offset value and the second offsetvalue, relatively easily, without any complicated operation orcalculation.

Moreover, since the waveform distortion occurring in the read signal iscorrected before the waveform equalization process is performed by thewaveform equalizing device, the waveform distortion hardly has or doesnot have an adverse effect on the waveform equalization process even ifthe waveform distortion occurs in the read signal read from therecording medium. More specifically, for example, it is possible topreferably prevent such a disadvantage that the waveform distortion isfurther emphasized or that the waveform distortion remains. In otherwords, correcting the waveform distortion can preferably prevent such adisadvantage that the long mark is misjudged to be another mark. Bythis, on the waveform equalizing device, the waveform equalizationprocess can be preferably performed on the read signal. As a result, therecord data can be preferably reproduced.

As described above, according to the information reproducing apparatusin the embodiment, even if the waveform distortion occurs, the waveformequalization can be excellently performed. As a result, even if thewaveform distortion occurs, the record data can be preferablyreproduced.

In one aspect of the embodiment of the information reproducing apparatusof the present invention, the first offset value is same as the secondoffset value.

According to this aspect, it is possible to relatively reduce a load forthe generation or calculation of the first offset value and the secondoffset value, as detailed later. In other words, it is possible togenerate or calculate the first offset value and the second offsetvalue, relatively easily, without any complicated operation orcalculation.

In another aspect of the embodiment of the information reproducingapparatus of the present invention, the first offset value is greaterthan the second offset value by a magnitude corresponding to a value seton the basis of at least one of (i) an asymmetry value which indicates ashift amount between an amplitude center of a read signal obtained byreading record data with the shortest run length of the read signal andan amplitude center of a read signal which provides a maximum amplitudeof a read signal; (ii) an entire β value which indicates an averagevalue of the amplitude center of the read signal; and (iii) a partial βvalue which indicates deviation between the amplitude center of the readsignal obtained by reading the record data with the shortest run lengthof the read signal and the amplitude center of the read signal obtainedby reading the record data with the second shortest run length of theread signal.

According to this aspect, it is possible to match the amplitude centerof the read signal obtained by reading the record data with the shortestrun length and the reference level (or zero level), in the read signalafter the second offset value is subtracted. Therefore, it is possibleto preferably perform the waveform equalization or the like after thesubtraction of the second offset value.

In an aspect of the information reproducing apparatus in which the firstoffset value is greater than the second offset value by a magnitudecorresponding to the value set in accordance with at least one of theasymmetry value, the entire β value, and the partial β value, asdescribed above, the first offset value may be greater than the secondoffset value by a magnitude corresponding to a value obtained bymultiplying the asymmetry value by an appearance probability, which doesnot consider the run length, of the record data with the shortest runlength with respect to the record data included in the read signal.

By virtue of such construction, by considering the actually occurringasymmetry value, it is possible to match the amplitude center of theread signal obtained by reading the record data with the shortest runlength and the reference level (or zero level), in the read signal afterthe second offset value is subtracted. Therefore, it is possible topreferably perform the waveform equalization or the like after thesubtraction of the second offset value.

Incidentally, the “appearance probability which does not consider therun length” in the embodiment is an appearance probability calculated byassigning an appearance frequency of 1 every time the record data witheach run length appears once, regardless of the length of the runlength. For example, if there are A record data with a run length of aT,B record data with a run length of bT, and C record data with a runlength of cT in the read signal in a certain range, the appearanceprobability of the record data with a run length of aT is A/(A+B+C), theappearance probability of the record data with a run length of bT isB/(A+B+C), and the appearance probability of the record data with a runlength of cT is C/(A+B+C).

In an aspect of the information reproducing apparatus in which the firstoffset value is greater than the second offset value by a magnitudecorresponding to the value set in accordance with at least one of theasymmetry value, the entire β value, and the partial β value, asdescribed above, the first offset value may be greater than the secondoffset value by a magnitude corresponding to a value obtained bymultiplying the entire β value by an appearance probability, which doesnot consider the run length, of the record data with the shortest runlength with respect to the record data included in the read signal.

By virtue of such construction, by considering the actually occurringentire β value, it is possible to match the amplitude center of the readsignal obtained by reading the record data with the shortest run lengthand the reference level (or zero level), in the read signal after thesecond offset value is subtracted. Therefore, it is possible topreferably perform the waveform equalization or the like after thesubtraction of the second offset value.

In an aspect of the information reproducing apparatus in which the firstoffset value is greater than the second offset value by a magnitudecorresponding to the value set in accordance with at least one of theasymmetry value, the entire β value, and the partial β value, asdescribed above, the first offset value may be greater than the secondoffset value by a magnitude corresponding to a value obtained bymultiplying the partial β value by an appearance probability, whichconsiders the run length, of the record data with the shortest runlength with respect to the record data included in the read signal.

By virtue of such construction, by considering the actually occurringpartial β value, it is possible to match the amplitude center of theread signal obtained by reading the record data with the shortest runlength and the reference level (or zero level), in the read signal afterthe second offset value is subtracted. Therefore, it is possible topreferably perform the waveform equalization or the like after thesubtraction of the second offset value.

Incidentally, the “appearance probability which considers the runlength” in the embodiment is an appearance probability calculated byassigning an appearance frequency weighted in accordance with the runlength, every time the record data with each run length appears once, inview of the length of the run length. For example, if there are A recorddata with a run length of aT, B record data with a run length of bT, andC record data with a run length of cT in the read signal in a certainrange, the appearance probability of the record data with a run lengthof aT is a×A/(a×A+b×B+c×C), the appearance probability of the recorddata with a run length of bT is b×B/(a×A+b×B+c×C), and the appearanceprobability of the record data with a run length of cT isc×C/(a×A+b×B+c×C).

In another aspect of the embodiment of the information reproducingapparatus of the present invention, the first offset value is greaterthan the second offset value by a magnitude corresponding to a value seton the basis of a positional relation between a reference level of theread signal and an amplitude center of a read signal obtained by readingrecord data with the shortest run length of the read signal.

According to this aspect, it is possible to match the amplitude centerof the read signal obtained by reading the record data with the shortestrun length and the reference level (or zero level), in the read signalafter the second offset value is subtracted. Therefore, it is possibleto preferably perform the waveform equalization or the like after thesubtraction of the second offset value.

In an aspect of the information reproducing apparatus in which the firstoffset value is greater than the second offset value by a magnitudecorresponding to the value set on the basis of the positional relationbetween the reference level of the read signal and the amplitude centerof the read signal obtained by reading record data with the shortest runlength of the read signal, as described above, the first offset valuemay be greater than the second offset value by a magnitude correspondingto a value indicating deviation between a reference level of the readsignal and an amplitude center of a read signal obtained by readingrecord data with the shortest run length of the read signal.

By virtue of such construction, it is possible to match the amplitudecenter of the read signal obtained by reading the record data with theshortest run length and the reference level (or zero level), in the readsignal after the second offset value is subtracted. Therefore, it ispossible to preferably perform the waveform equalization or the likeafter the subtraction of the second offset value.

In another aspect of the embodiment of the information reproducingapparatus of the present invention, if reflectance of a mark is smallerthan reflectance of a space, at least one of the first offset value andthe second offset value is less than a difference between a maximumvalue of a signal level of the long mark in which the waveformdistortion occurs (and more specifically, the signal level of the peakof the waveform distortion) and a reference level of the read signal.

According to this aspect, it is possible to generate or calculate thefirst offset value and the second offset value, relatively easily, whilepreferably preventing such a disadvantage that the record data with arelatively short run length is misjudged to be the waveform distortion.

In another aspect of the embodiment of the information reproducingapparatus of the present invention, if reflectance of a mark is smallerthan reflectance of a space, at least one of the first offset value andthe second offset value is a half of a difference between a maximumvalue of a signal level of the long mark in which the waveformdistortion occurs (and more specifically, the signal level of the peakof the waveform distortion) and a reference level of the read signal.

According to this aspect, it is possible to generate or calculate thefirst offset value and the second offset value, relatively easily, whilepreferably preventing such a disadvantage that the record data with arelatively short run length is misjudged to be the waveform distortion.

In another aspect of the embodiment of the information reproducingapparatus of the present invention, if reflectance of a mark is greaterthan reflectance of a space, at least one of the first offset value andthe second offset value is less than a difference between a minimumvalue of a signal level of the long mark in which the waveformdistortion occurs (and more specifically, the signal level of the peakof the waveform distortion) and a reference level of the read signal.

According to this aspect, it is possible to generate or calculate thefirst offset value and the second offset value, relatively easily, whilepreferably preventing such a disadvantage that the record data with arelatively short run length is misjudged to be the waveform distortion.

In another aspect of the embodiment of the information reproducingapparatus of the present invention, if reflectance of a mark is greaterthan reflectance of a space, at least one of the first offset value andthe second offset value is a half of a difference between a minimumvalue of a signal level of the long mark in which the waveformdistortion occurs (and more specifically, the signal level of the peakof the waveform distortion) and a reference level of the read signal.

According to this aspect, it is possible to generate or calculate thefirst offset value and the second offset value, relatively easily, whilepreferably preventing such a disadvantage that the record data with arelatively short run length is misjudged to be the waveform distortion.

In another aspect of the embodiment of the information reproducingapparatus of the present invention, the waveform equalizing device isprovided with: an amplitude limiting device for limiting an amplitudelevel of the read signal in which the waveform distortion is corrected,by a predetermined amplitude limit value, thereby obtaining an amplitudelimit signal; and a filtering device for performing a high-frequencyemphasis filtering process on the amplitude limit signal, therebyobtaining an equalization-corrected signal.

According to this aspect, by the operation of the amplitude limitingdevice, the amplitude level of the read signal in which the waveformdistortion is corrected (hereinafter referred to as a“distortion-corrected signal”, as occasion demands) is limited.Specifically, with respect to a signal component of thedistortion-corrected signal whose amplitude level is greater than theupper limit of the amplitude limit value or is less than the lower limitof the amplitude limit value, the amplitude level is limited to theupper limit or the lower limit of the amplitude limit value. On theother hand, with respect to a signal component of thedistortion-corrected signal whose amplitude level is less than or equalto the upper limit of the amplitude limit or is greater than or equal tothe lower limit of the amplitude limit value, its amplitude level is notlimited. The distortion-corrected signal in which the amplitude level islimited as described above is outputted to the filtering device as theamplitude limit signal. On the filtering device, the high-frequencyemphasis process is performed on the amplitude limit signal. As aresult, the equalization-corrected signal is obtained. Then, forexample, a binary process, a decoding process, and the like areperformed on the equalization-corrected signal. By this, a process ofreproducing the record data (e.g. video data, audio data, and the like)recorded on the recording medium can be performed.

By this, on the filtering device, it is possible to limit or control theoccurrence of the dispersion (i.e. jitter) of the read signal (or itssample values), and as a result, it is possible to perform thehigh-frequency emphasis on the read signal without any intersymbolinterference.

Moreover, since the waveform distortion occurring in the read signal iscorrected before the waveform equalization process is performed by thewaveform equalizing device, the waveform distortion hardly has or doesnot have an adverse effect on the amplitude limit and the high-frequencyemphasis filtering even if the waveform distortion occurs in the readsignal read from the recording medium. More specifically, for example,it is possible to preferably prevent such a disadvantage that thewaveform distortion is further emphasized, which is caused by that thewaveform distortion is less than or equal to the upper limit of theamplitude limit value or is greater than or equal to the lower limit ofthe amplitude limit value. As a result, for example, it is possible topreferably prevent such a disadvantage that the long mark is misjudgedto be another mark. By this, on the limit equalizer (i.e. the amplitudelimiting device and the filtering device), the high-frequency emphasiscan be preferably performed on the read signal.

In another aspect of the embodiment of the information reproducingapparatus of the present invention, the offset adding device adds thefirst offset value and the offset subtracting device subtracts thesecond offset value (i) if an error correction of the read signal (morespecifically, an error correction of the record data obtained from theread signal) cannot be performed, (ii) if an error rate of the readsignal is greater than or equal to a predetermined threshold value, or(iii) if a read signal corresponding to synchronization data cannot beread, the synchronization data being used to read user data included inrecord data, the synchronization data being included in the record data.

According to this aspect, by selectively adding the first offset valueand subtracting the second offset value in the aforementioned cases, itis possible to receive the aforementioned various effects while reducinga load of the information reproducing apparatus.

Moreover, it is possible to relatively easily realize the optimum offsetvalue by adding the first offset value and subtracting the second offsetvalue (and particularly changing the offset value as occasion demands)while monitoring whether or not the error correction is unable to bemade, whether or not the error rate of the read signal is greater thanor equal to the predetermined threshold value, or whether or not thesynchronization data can be read, as occasion demands.

In another aspect of the embodiment of the information reproducingapparatus of the present invention, the correcting device corrects thewaveform distortion (i) if an error correction of the read signal cannotbe performed, (ii) if an error rate of the read signal is greater thanor equal to a predetermined threshold value, or (iii) if a read signalcorresponding to synchronization data cannot be read, thesynchronization data being used to read user data included in recorddata, the synchronization data being included in the record data.

According to this aspect, by selectively correcting the waveformdistortion in the aforementioned cases, it is possible to receive theaforementioned various effects while reducing the load of theinformation reproducing apparatus.

In particular, as opposed to the recording medium which allows onlysequential recording, various recording statuses are mixed in therecording medium which allows random recording. In this case, it isnecessary to read the read signal in which the waveform distortion isdiscontinuously or discretely distributed or not distributed, or to readthe read signal which has various signal levels. Therefore, byreproducing the record data without correcting the waveform distortionin a normal case and by reproducing the record data while selectivelycorrecting the waveform distortion in the aforementioned cases, it ispossible to receive the aforementioned various effects while reducingthe load of the information reproducing apparatus.

In another aspect of the embodiment of the information reproducingapparatus of the present invention, the long mark is a mark whose signallevel is maximum amplitude.

According to this aspect, it is possible to preferably correct thewaveform distortion occurring in the read signal corresponding to thelong mark.

(Embodiment of Information Reproducing Method)

An embodiment of the information reproducing method of the presentinvention is an information reproducing method provided with: an offsetadding process of adding a first offset value which can be set to bevariable, to a read signal read from a recording medium; a correctingprocess of correcting waveform distortion occurring in a read signalcorresponding to at least a long mark, of the read signal to which thefirst offset value is added by the offset adding process; an offsetsubtracting process of subtracting a second offset value which can beset to be variable, from the read signal in which the waveformdistortion is corrected; and a waveform equalizing process of performinga waveform equalization process on the read signal in which the secondoffset value is subtracted.

According to the embodiment of the information reproducing method of thepresent invention, it is possible to receive the same various effects asthose that can be received by the aforementioned embodiment of theinformation reproducing apparatus of the present invention.

Incidentally, in response to the various aspects in the aforementionedembodiment of the information reproducing apparatus of the presentinvention, the embodiment of the information reproducing method of thepresent invention can also adopt various aspects.

(Embodiment of Computer Program)

An embodiment of the computer program of the present invention is acomputer program for reproduction control and for controlling a computerprovided in an information reproducing apparatus provided with: anoffset adding device for adding a first offset value which can be set tobe variable, to a read signal read from a recording medium; a correctingdevice for correcting waveform distortion occurring in a read signalcorresponding to at least a long mark, of the read signal to which thefirst offset value is added by the offset adding device; an offsetsubtracting device for subtracting a second offset value which can beset to be variable, from the read signal in which the waveformdistortion is corrected; and a waveform equalizing device for performinga waveform equalization process on the read signal in which the secondoffset value is subtracted (i.e. the aforementioned embodiment of theinformation reproducing apparatus of the present invention (includingits various aspects)), the computer program making the computer functionas at least one portion of the offset adding device, the correctingdevice, the offset subtracting device, and the waveform equalizingdevice.

According to the embodiment of the computer program of the presentinvention, the aforementioned embodiment of the information reproducingapparatus of the present invention can be relatively easily realized asa computer reads and executes the computer program from a programstorage device, such as a ROM, a CD-ROM, a DVD-ROM, and a hard disk, oras it executes the computer program after downloading the programthrough a communication device.

Incidentally, in response to the various aspects in the aforementionedembodiment of the information reproducing apparatus of the presentinvention, the embodiment of the computer program of the presentinvention can also employ various aspects.

An embodiment of the computer program product of the present inventionis a computer program product in a computer-readable medium for tangiblyembodying a program of instructions executable by a computer provided inan information reproducing apparatus provided with: an offset addingdevice for adding a first offset value which can be set to be variable,to a read signal read from a recording medium; a correcting device forcorrecting waveform distortion occurring in a read signal correspondingto at least a long mark, of the read signal to which the first offsetvalue is added by the offset adding device; an offset subtracting devicefor subtracting a second offset value which can be set to be variable,from the read signal in which the waveform distortion is corrected; anda waveform equalizing device for performing a waveform equalizationprocess on the read signal in which the second offset value issubtracted (i.e. the aforementioned embodiment of the informationreproducing apparatus of the present invention (including its variousaspects)), the computer program making the computer function as at leastone portion of the offset adding device, the correcting device, theoffset subtracting device, and the waveform equalizing device.

According to the embodiment of the computer program product of thepresent invention, the aforementioned embodiment of the informationreproducing apparatus of the present invention can be embodiedrelatively readily, by loading the computer program product from arecording medium for storing the computer program product, such as a ROM(Read Only Memory), a CD-ROM (Compact Disc-Read Only Memory), a DVD-ROM(DVD Read Only Memory), a hard disk or the like, into the computer, orby downloading the computer program product, which may be a carrierwave, into the computer via a communication device. More specifically,the computer program product may include computer readable codes tocause the computer (or may comprise computer readable instructions forcausing the computer) to function as the aforementioned embodiment ofthe information reproducing apparatus of the present invention.

Incidentally, in response to the various aspects in the aforementionedembodiment of the information reproducing apparatus of the presentinvention, the embodiment of the computer program product of the presentinvention can also employ various aspects.

The operation and other advantages of the present invention will becomemore apparent from the examples explained below.

As explained above, according to the embodiment of the informationreproducing apparatus of the present invention, it is provided with theoffset adding device, the correcting device, the offset subtracting, andthe waveform equalizing device. According to the embodiment of theinformation reproducing method of the present invention, it is providedwith the offset adding process, the correcting process, the offsetsubtracting process, and the waveform equalizing process. According tothe embodiment of the computer program of the present invention, itmakes a computer function as the embodiment of the informationreproducing apparatus of the present invention. Therefore, it ispossible to preferably reproduce the data even if the waveformdistortion occurs.

EXAMPLES

Hereinafter, an example of the present invention will be described onthe basis of the drawings.

(1) Basic Structure

Firstly, with reference to FIG. 1, an example of the informationreproducing apparatus of the present invention will be described. FIG. 1is a block diagram conceptually showing the basic structure of theinformation reproducing apparatus in the example.

As shown in FIG. 1, an information reproducing apparatus 1 in theexample is provided with a spindle motor 10, a pickup (PU) 11, a HPF(High Pass Filter) 12, an A/D converter 13, a pre-equalizer 14, a limitequalizer 15, a binary circuit 16, a decoding circuit 17, a waveformdistortion correction circuit 18, an adder 19-1, an offset generationcircuit 19-2, and a subtracter 19-3.

The pickup 11 photoelectrically converts reflected light when a laserbeam LB is irradiated to a recording surface of an optical disc 100rotated by the spindle motor 10, thereby generating a read signalR_(RF).

The HPF 12 removes a low-frequency component of the read signal R_(RF)outputted from the pickup, and it outputs a resulting read signalR_(HC), to the A/D converter 13.

The A/D converter 13 samples the read signal in accordance with asampling clock outputted from a PLL (Phased Lock Loop) not illustratedor the like, and it outputs a resulting read sample value series RS tothe pre-equalizer 14.

The pre-equalizer 14 removes intersymbol interference based ontransmission characteristics in an information reading system, which isformed of the pickup 11 and the optical disc 100, and it outputs aresulting read sample value series RS_(C) to the adder 19-1.

The adder 19-1 constitutes one specific example of the “offset addingdevice” of the present invention. The adder 19-1 adds an offset valueOFS generated on the offset generation circuit, to the read sample valueseries RS_(C) outputted from the pre-equalizer 14. The read sample valueseries RS_(C) with the offset value OFS added is outputted to thewaveform distortion correction circuit 18.

The offset generation circuit 19-2 constitutes one specific example ofthe “offset adding device” and the “offset subtracting device” of thepresent invention and generates the offset value OFS. Incidentally, theoffset value OFS will be detailed later (refer to FIG. 13 and subsequentdrawings).

The waveform distortion correction circuit 18 constitutes one specificexample of the “correcting device” of the present invention. Thewaveform distortion correction circuit 18 corrects waveform distortionoccurring in the read sample value series RS_(C) (i.e. waveformdistortion occurring in the read signal R_(RF)). A resultingdistortion-corrected read sample value series RS_(CAM) is outputted tothe subtracter 19-3.

Incidentally, a specific structure and operations of the waveformdistortion correction circuit 18 will be detailed later (refer to FIG. 6and subsequent drawings).

The subtracter 19-3 constitutes one specific example of the “offsetsubtracting device” of the present invention. The subtracter 19-3subtracts the offset value OFS generated on the offset generationcircuit 19-2, from the distortion-corrected read sample value seriesRS_(CAM). The distortion-corrected read sample value series RS_(CAM)with the offset value OFS subtracted is outputted to the limit equalizer15.

The limit equalizer 15 performs a high-frequency emphasis process on thedistortion-corrected read sample value series RS_(CAM) withoutincreasing the intersymbol interference, and it outputs a resultinghigh-frequency emphasized read sample value series RS_(H) to the binarycircuit 16.

The binary circuit 16 performs a binary process on the high-frequencyemphasized read sample value series RS_(H), and it outputs a resultingbinary signal to the decoding circuit 17.

The decoding circuit 17 performs a decoding process or the like on thebinary signal, and it outputs a resulting reproduction signal toexternal reproduction equipment, such as a display and a speaker. As aresult, data recorded on the optical disc 100 (e.g. video data, audiodata, and the like) is reproduced.

Next, with reference to FIG. 2, the more detailed structure of the limitequalizer 15 will be described. FIG. 2 is a block diagram conceptuallyshowing the structure of the limit equalizer 15. As shown in FIG. 2, thelimit equalizer 15 is provided with an amplitude limit value settingblock 151, an amplitude limit block 152, and a high-frequency emphasisblock 153.

The amplitude limit value setting block 151 sets the upper limit and thelower limit of an amplitude limit value which are used on the amplitudelimit block 152, on the basis of the distortion-corrected read samplevalue series RS_(CAM). The amplitude limit block 152 performs anamplitude limit process on the distortion-corrected read sample valueseries RS_(CAM), on the basis of the upper limit and the lower limit ofthe amplitude limit value which are set on the amplitude limit valuesetting block 151. A sample value series RS_(LIM) on which the amplitudelimit process is performed is outputted to the high-frequency emphasisblock 153. The high-frequency emphasis block 153 performs a filteringprocess for emphasizing high frequencies, on the sample value seriesRS_(LIM) on which the amplitude limit process is performed. As a result,the high-frequency emphasized read sample value series RS_(H) isobtained.

More specifically, a reference sample timing detection circuit 1511detects reference sample timing, on the basis of thedistortion-corrected read sample value series RS_(CAM). The detectedreference sample timing is outputted to a sample hold circuit 1514through a delayer 1512 for providing a one-clock delay and an OR circuit1513. On the sample hold circuit 1514, a sample value series RS_(P)outputted from an interpolation filter 1522 is sampled and held inaccordance with the reference sample timing outputted through thedelayer 1512 and the OR circuit 1513.

Incidentally, the interpolation filter 1522 performs an interpolationprocess on the distortion-corrected read sample value series RS_(CAM),thereby generating an interpolated sample value series which is obtainedwhen the read signal R_(RF) read from the optical disc 100 is sampled inthe middle timing of the clock timing based on the sampling clock usedon the A/D converter 14. The generated interpolated sample value seriesis included in the distortion-corrected read sample value seriesRS_(CAM), and it is outputted to a limiter 1523 and the sample holdcircuit 1514, as the sample value series RS_(P).

In the read sample value series RS_(P) sampled and held, a referencelevel Rf is subtracted on a subtracter 1515. Incidentally, if a zerolevel is used as the reference level Rf, Rf=0. The subtraction result isoutputted to an averaging circuit 1516. The averaging circuit 1516calculates an average value of an absolute value of each of samplevalues. The calculated average value of sample values is set as theupper limit and the lower limit of the amplitude limit value.Specifically, a value obtained by adding the average value to thereference level is set as the upper limit of the amplitude limit value,and a value obtained by subtracting the average value from the referencelevel is set as the lower limit of the amplitude limit value. If thezero level is used as the reference level, a value obtained by providinga positive sign for the calculated average value of sample values is setas the upper limit of the amplitude limit value, and a value obtained byproviding a negative sign for the calculated average value of samplevalues is set as the lower limit of the amplitude limit value. In thefollowing explanation, for convenience of explanation, the zero level isused as the reference level Rf.

Specifically, with reference to FIG. 3, an explanation will be given onthe upper limit and the lower limit of the amplitude limit value set onthe amplitude limit value setting block 151. FIG. 3 is a waveform chartconceptually showing an operation of setting the upper limit and thelower limit of the amplitude limit value, on the distortion-correctedread sample value series RS_(CAM).

FIG. 3 shows the read signal R_(RF) obtained by reading data with arelatively short run length (specifically, data with run lengths of 2 T,3 T, and 4 T if the optical disc 100 is a Blu-ray Disc) of the readsignal; and its distortion-corrected read sample value series RS_(CAM).As shown in FIG. 3, an average value L of absolute values ofinterpolated sample values (sample values generated on the interpolationfilter 1522) located before (i.e. before in terms of time) a zero crosspoint and interpolated sample values located after (i.e. after in termsof time) the zero cross point is set as the absolute value of the uppervalue and the lower value of the amplitude limit value. In other words,the upper limit of the amplitude limit value is set as L, and the lowerlimit of the amplitude limit value is set as −L.

Back in FIG. 2 again, the limiter 1523 performs amplitude limit on thesample value series RS_(P) on the basis of the upper limit and the lowerlimit which are set on the amplitude limit value setting block 151.Specifically, if a sample value included in the sample value seriesRS_(P) is less than the upper limit L and greater than the lower limit−L, the sample value is outputted as the sample value series RS_(LIM) asit is. On the one hand, if a sample value included in the sample valueseries RS_(P) is greater than or equal to the upper limit L, the upperlimit L is outputted as the sample value series RS_(LIM). On the otherhand, if a sample value included in the sample value series RS_(P) isless than or equal to the upper limit −L, the lower limit −L isoutputted as the sample value series RS_(LIM).

The high-frequency emphasis block 153 increases the signal level of onlythe sample value series RS_(LIM) corresponding to data with the shortestrun length (e.g. the data with a run length of 3 T if the optical disc100 is a DVD, and the data with a run length of 2 T if the optical disc100 is a Blu-ray Disc) in the sample value series RS_(UM).

Specifically, the sample value series RS_(LIM) inputted to thehigh-frequency emphasis block 153 is inputted to coefficient multipliers1535 and 1538 having a multiplier coefficient of −k and coefficientmultipliers 1536 and 1537 having a multiplier coefficient of k, as it isor through delayers 1532, 1533, and 1534 for providing a one-clockdelay. The outputs of the coefficient multipliers 1535, 1536, 1537, and1538 are added on an adder 1539. The addition result, a high-frequencyread sample value RS_(HIG), is added to the distortion-corrected readsample value series RS_(CAM) which is inputted to the adder 1531 throughthe delayer 1530 for providing a three-clock delay, on the adder 1531.As a result, the high-frequency emphasized read sample value seriesRS_(H) is obtained.

Now, with reference to FIG. 4, an operation of obtaining thehigh-frequency emphasized read sample value series RS_(H) will bedescribed in more detail. FIG. 4 are waveform charts conceptuallyshowing the operation of obtaining the high-frequency emphasized readsample value series RS_(H), on the distortion-corrected read samplevalue series RS_(CAM).

As shown in FIG. 4( a), the high-frequency read sample value RS_(HIG)outputted from the adder 1531 is calculated on the basis of the samplevalues at respective time points D (−1.5), D(−0.5), D(0.5), and D(1.5)in the sample value series RS_(CAM). Specifically, if the sample valuesat the respective time points D (−1.5), D(−0.5), D(0.5), and D(1.5) inthe sample value series RS_(LIM) are Sip(−1), Sip(0), Sip(1), andSip(2), then, RS_(HIG)=(−k)×Sip(−1)+k×Sip(0)+k×Sip(1)+(−k)×Sip(2).

At this time, as shown in FIG. 4( b), the sample values Sip(−1) andSip(0) at the respective time points D(−1.5) and D(−0.5) correspondingto the data with a run length of 2 T are substantially equal to eachother. Moreover, the sample values Sip(1) and Sip(2) at the respectivetime points D(0.5) and D(1.5) corresponding to the data with a runlength of 2 T are substantially equal to each other.

Moreover, as shown in FIG. 4( c), the sample values Sip(−1) and Sip(0)at the respective time points D(−1.5) and D(−0.5) corresponding to thedata with each of run lengths of 3 T and 4 T are both the upper limit Lof the amplitude limit value, due to the amplitude limit by theamplitude limit block 152. In the same manner, the sample values Sip(1)and Sip(2) at the respective time points D(0.5) and D(1.5) correspondingto the data with each of run lengths of 3 T and 4 T are both the lowerlimit −L of the amplitude limit value, due to the amplitude limit by theamplitude limit block 152. In other words, the dispersion of the samplevalues before and after the reference sample point is forciblycontrolled.

Thus, even if the value of the coefficient k is increased on thecoefficient multipliers 1535, 1536, 1537, and 1538 in order to increasethe high-frequency emphasis, the high-frequency read sample valueRS_(HIG) obtained at the zero cross point D(0) is kept constant.Therefore, the intersymbol interference does not occur. As describedabove, according to the information reproducing apparatus 1 providedwith the limit equalizer 15, the dispersion of the sample values beforeand after the zero cross point in the read signal, which causes theintersymbol interference, is forcibly controlled in performing thehigh-frequency emphasis. Thus, even if the sufficient high-frequencyemphasis is performed on the high-frequency emphasis block 153, theintersymbol interference does not occur.

In particular, in the information reproducing apparatus 1 in theexample, the waveform distortion is corrected after the offset value OFSis added to the read signal R_(RF) (and more specifically, the readsample value series RS_(C)), and then, the added offset value issubtracted, and then, the amplitude limit and the high-frequencyemphasis are performed on the limit equalizer 15. Hereinafter, adetailed explanation will be given on specific examples of the offsetvalue OFS and the waveform distortion correction.

(2) Waveform Distortion

Firstly, with reference to FIG. 5 and FIG. 6, the waveform distortionwill be described. FIG. 5 are waveform charts conceptually showing afirst example of waveform distortion. FIG. 6 are waveform chartsconceptually showing a second example of waveform distortion.

As shown in FIG. 5( a), the waveform distortion indicates a differencebetween a proper signal level to be taken and a signal level thatactually appears in the read signal R_(RF). The waveform distortion isquantitatively defined by a waveform distortion amount D for the maximumamplitude A of the read signal R_(RF), and a waveform distortion amountD′ which is a signal level from the zero level to the peak of thewaveform distortion. In FIG. 5( a), a thick dashed line denotes theproper signal level to be taken when there is no waveform distortion. Ifthere is no waveform distortion, the waveform distortion amount D isobviously zero.

Incidentally, the waveform distortion shown in FIG. 5( a) shows suchwaveform distortion that the signal level in a middle portion ischanged, compared to the signal level in a front edge portion and a rearedge portion of the read signal R_(RF). Apart from such waveformdistortion, there can be such waveform distortion that the signal levelin the front edge portion and the middle portion is changed, compared tothe signal level in the rear edge portion of the read signal R_(RF) asshown in FIG. 5( b); and such waveform distortion that the signal levelin the middle edge portion and the rear portion is changed, compared tothe signal level in the front edge portion of the read signal R_(RF) asshown in FIG. 5( c). For any waveform distortion, the structure andoperation described later can be obviously adopted.

Moreover, in FIG. 5( a) to FIG. 5( c), an explanation was given on thewaveform distortion occurring on the optical disc 100 in which thereflectance of the laser beam LB is reduced by forming the marks. Inother words, an explanation was given on the example in which thewaveform distortion occurs such that the signal level unintentionallyincreases in the signal level which is the zero level or less. However,as shown in FIG. 6( a), there can be also the waveform distortionoccurring on the optical disc 100 (or so-called low-to-high disc) inwhich the reflectance of the laser beam LB is increased by recording thedata, as in an optical disc such as a Blu-ray disc in which a pigmentedfilm is used as a recording layer. In other words, such waveformdistortion can occur that the signal level unintentionally reduces inthe signal level which is the zero level or more. Incidentally, in thecase where such waveform distortion can occur that the signal levelunintentionally reduces in the signal level which is the zero level ormore, there can be such waveform distortion that the signal level in thefront edge portion and the middle portion is changed, compared to thesignal level in the rear edge portion of the read signal R_(RF), asshown in FIG. 6( b), as in the case where such waveform distortionoccurs that the signal level unintentionally reduces as shown in FIG. 5(b), in the signal level which is the zero level or more. Moreover, therecan be also such waveform distortion that the signal level in the middleportion and the rear edge portion is changed, compared to the signallevel in the front edge portion of the read signal R_(RF), as shown inFIG. 6( c), as in the case where such waveform distortion occurs thatthe signal level unintentionally reduces shown in FIG. 5( c).

Moreover, in the example, it is preferable to focus on the waveformdistortion which occurs in the read signal corresponding to the recordmark with a relatively long run length (hereinafter referred to as a“long mark”: e.g. data with run lengths of 7 T to 11 T or 14 T if theoptical disc 100 is a DVD, and data with run lengths of 6 T to 9 T ifthe optical disc 100 is a Blu-ray Disc). Alternatively, with emphasis onthe importance of synchronization data (i.e. sync data), it ispreferable to focus on the waveform distortion which occurs in the readsignal corresponding to the mark corresponding to the synchronizationdata (e.g. data with a run length of 14 T if the optical disc 100 is aDVD, and data with a run length of 9 T if the optical disc 100 is aBlu-ray Disc).

(3) Operation Example

Next, with reference to FIG. 7 to FIG. 9, an explanation will be givenon a specific operation example of the adder 19-1, the offset additioncircuit 19-2, the subtracter 19-3, and the waveform distortioncorrection circuit 18. FIG. 7 is a flowchart conceptually showing a flowof operations of the adder 19-1, the offset addition circuit 19-2, thesubtracter 19-3, and the waveform distortion correction circuit 18. FIG.8 is a block diagram conceptually showing the structure of the waveformdistortion correction circuit 18. FIG. 9 is a waveform chartconceptually showing an operation of correcting the waveform distortionby the waveform distortion correction circuit 18, on the sample valueseries RS_(C).

As shown in FIG. 7, firstly, an operation of reproducing data recordedon the optical disc 100 is performed (step S101).

In the reproduction operation, it is sequentially judged whether or nota symbol error rate (S ER) is greater than or equal to a predeterminedthreshold value, or whether or not error correction using an ECC (ErrorCorrection Code) or the like is unable to be performed, or whether ornot the synchronization data is unable to be read (step S102). Here, thepredetermined threshold value is preferably set on the basis of whetheror not the preferable reproduction is performed. Specifically, it ispreferable to set the value of the symbol error rate which does notallow the preferable reproduction operation (e.g. approximately 0.001 ormore), as the predetermined threshold value.

As a result of the judgment in the step S102, if it is judged that thesymbol error rate is not greater than or equal to the threshold value,and that the error correction is not unable to be performed, and thatthe synchronization data is not unable to be read (the step S102: No),the operational flow goes to a step S112.

On the other hand, as a result of the judgment in the step S102, if itis judged that the symbol error rate is greater than or equal to thethreshold value, or that the error correction is unable to be performed,or that the synchronization data is unable to be read (the step S102:Yes), then, the waveform distortion of the long mark is measured (stepS103). Here, a waveform distortion ratio (i.e. D/A×100) which indicatesa ratio of the waveform distortion amount D (or D′) to the maximumamplitude A of the read signal R_(RF).

Then, it is judged whether or not the waveform distortion is greaterthan or equal to a predetermined value (step S104). For example, it isjudged whether or not the waveform distortion ratio is greater than orequal to approximately 30%.

As a result of the judgment in the step S104, if it is judged that thewaveform distortion is not greater than or equal to the predeterminedvalue (e.g. that the waveform distortion ratio is less than or equal toapproximately 30%) (the step S104: No), the operational flow goes to thestep S112.

On the other hand, as a result of the judgment in the step S104, if itis judged that the waveform distortion is greater than or equal to thepredetermined value (e.g. that the waveform distortion ratio is greaterthan or equal to approximately 30%) (the step S104: Yes), by theoperation of the offset generation circuit 19-2, it is judged ordetermined whether or not the offset value OFS is to be added to theread signal R_(RF) (and more specifically the read sample value seriesRS_(C)) (step S105). The judgment may be performed, for example, on thebasis of a deviation ratio (or rate) of the amplitude center of the readsignal R_(RF) corresponding to the record data with the shortest runlength, with respect to the amplitude center (i.e. the reference level,and the zero level in the example) of the read signals R_(RF)corresponding to the respective record data with all types of runlengths (e.g. the record data with each of run lengths of 3 T to 11 Tand 14 T if the optical disc 100 is a DVD, and the record data with eachof run lengths of 2 T to 9 T if the optical disc 100 is a Blu-ray Disc).For example, it may be judged that the offset value OFS is not added ifthe deviation ratio is substantially 0, and it may be judged that theoffset value OFS is added if the deviation ratio is not substantially 0.Alternatively, the judgment may be performed on the same judgmentcriterion as that of the judgment in the step S102 described above. Forexample, it may be judged that the offset value OFS is not added if thesymbol error rate is not greater than or equal to a predeterminedthreshold value, and the error correction is not unable to be performed,and the synchronization data is not unable to be read, and it may bejudged that the offset value OFS is added if the symbol error rate isgreater than or equal to the predetermined threshold value, or the errorcorrection is unable to be performed, or the synchronization data isunable to be read.

As a result of the judgment in the step S105, if it is judged that theoffset value OFS is not to be added (the step S105: No), the offsetvalue OFS is set to be zero (step S107), and then the operational flowgoes to a step S108.

On the other hand, as a result of the judgment in the step S105, if itis judged that the offset value OFS is to be added (the step S105: Yes),the offset value OFS is generated by the operation of the offsetgeneration circuit 19-2 (step S106), and the operational flow goes tothe step S108.

The offset value OFS generated here is preferably less than a difference(i.e. the waveform distortion amount D′) between the reference level andthe maximum value of the signal level of the long mark in which thewaveform distortion occurs, if the waveform distortion occurs as shownin FIG. 5( a) to FIG. 5( c). More preferably, it is preferably less thana half of the difference (i.e. the waveform distortion amount D′)between the reference level and the maximum value of the signal level ofthe long mark in which the waveform distortion occurs. In other words,offset is generated in a direction of the waveform distortionapproaching the reference level.

The offset value OFS generated here is preferably less than a difference(i.e. the waveform distortion amount −D′) between the reference leveland the minimum value of the signal level of the long mark in which thewaveform distortion occurs, if the waveform distortion occurs as shownin FIG. 6( a) to FIG. 6( c). More preferably, it is preferably less thana half of the difference (i.e. the waveform distortion amount −D′)between the reference level and the minimum value of the signal level ofthe long mark in which the waveform distortion occurs. In other words,the offset is generated in the direction of the waveform distortionapproaching the reference level.

Then, the offset value OFS, which is generated by the operation of theadder 19-1 in the step S106 or S107, is added to the read signal R_(RF)(and more specifically, the read sample value series RS_(C)) (stepS108).

Then, a waveform distortion correction condition, such as a correctionlevel and a correction range for the waveform distortion, is set (stepS109). The waveform distortion correction condition will be detailedlater (refer to FIG. 9 and the like).

Then, the waveform distortion of the long mark is corrected on the basisof the waveform distortion correction condition set in the step S109(step S110).

Then, the offset value OFS added in the step S108 is subtracted from thedistortion-corrected read sample value series RS_(CAM) (step S111);namely, the same offset value as that added in the step S108 issubtracted from the distortion-corrected read sample value seriesRS_(CAM).

Then, it is judged whether or not the reproduction operation is to beended (step S112), and if the reproduction operation is not to be ended(the step S112: No), the operational flow returns to the step S101, andthe operations after the step S101 are repeated again.

Of the operations shown in FIG. 7, the operation about the correction ofthe waveform distortion is performed mainly by the waveform distortioncorrection circuit 18. Now, the specific circuit structure of thewaveform distortion correction circuit will be described.

As shown in FIG. 8, the waveform distortion correction circuit 18 isprovided with a delay adjustment circuit 181, a distortion correctionvalue detection circuit 182, a mark/space length detection circuit 183,a timing generation circuit 184, and a selector 185.

The read sample value series RS_(C) outputted from the pre-equalizer 14is outputted to each of the delay adjustment circuit 181, the distortioncorrection value detection circuit 182, and the mark/space lengthdetection circuit 183.

The distortion correction value detection circuit 182 holds a samplevalue S(k) at a time point which is a time corresponding to minT afterthe zero cross point and outputs it as a distortion correction value amdto the selector 185.

Incidentally, minT indicates the read signal R_(RF) corresponding to therecord data with the shortest run length (and more specifically the readsample value series RS_(C) corresponding to the read signal R_(RF)). Forexample, if the optical disc 100 is a DVD, the minT indicates the readsignal R_(RF) corresponding to the record data with a run length of 3 T.For example, if the optical disc 100 is a Blu-ray Disc, the minTindicates the read signal R_(RF) corresponding to the record data with arun length of 2 T.

Moreover, the delay adjustment circuit 181 sets a delay amountcorresponding to the longest run length of the record data and outputsthe read sample value series RS_(C) to the selector 185 in desiredtiming. Specifically, if the optical disc 100 is a Blu-ray Disc, thedelay adjustment circuit 181 sets a delay amount corresponding to thelongest run length of 9 T, and if the optical disc 100 is a DVD, thedelay adjustment circuit 181 sets a delay amount corresponding to thelongest run length of 14 T.

The mark/space length detection circuit 183 detects a mark/space lengthby detecting an interval between the zero cross points, the number ofcontinuous coded bits, and the like. The detection result is outputtedto the timing generation circuit 184.

The timing generation circuit 184 generates a timing signal SW on thebasis of the mark/space length detected on the mark/space lengthdetection circuit 183 and outputs the generated timing signal SW to theselector 185.

Specifically, the timing generation circuit 184 generates a high-leveltiming signal SW (SW=1) (i) if the mark/space length detected on themark/space length detection circuit 183 is the long mark which is atarget of the waveform distortion correction and (ii) in a periodbetween a time point T1 which is at least a time corresponding to theminT after a first zero cross point and a time point T2 which is thetime corresponding to the minT before a second zero cross point locatednext to the first zero cross point, and the timing generation circuit184 outputs the generated timing signal SW to the selector 185. On theother hand, the timing generation circuit 184 generates a low-leveltiming signal SW (SW=0) (i) if the mark/space length detected on themark/space length detection circuit 183 is a mark other than the longmark which is a target of the waveform distortion correction or (ii) ina period other than the period between the time point T1 which is atleast a time corresponding to the minT after the first zero cross pointand the time point T2 which is the time corresponding to the minT beforethe second zero cross point located next to the first zero cross point,and the timing generation circuit 184 outputs the generated timingsignal SW to the selector 185.

If the high-level timing signal SW is outputted from the timinggeneration circuit 184, the selector 185 outputs the distortioncorrection value amd outputted from the distortion correction valuedetection circuit 182, to the limit equalizer 15 as thedistortion-corrected read sample value series RS_(CAM). On the otherhand, if the low-level timing signal SW is outputted from the timinggeneration circuit 184, the selector 185 outputs the read sample valueseries RS_(C) outputted from the delay adjustment circuit 181, to thelimit equalizer 15 as the distortion-corrected read sample value seriesRS_(CAM).

Incidentally, the waveform distortion correction condition set in thestep S109 in FIG. 7 substantially corresponds to the distortioncorrection value amd detected on the distortion correction valuedetection circuit 182 and the timing signal SW generated on the timinggeneration circuit 184.

The operations by the waveform distortion correction circuit 18 will bedescribed more clearly on a waveform chart showing the sample valueseries RS_(C).

As shown in FIG. 9, in the period between the time point T1 which is atleast the time corresponding to the minT after the first zero crosspoint and the time point T2 which is the time corresponding to the minTbefore the second zero cross point located next to the first zero crosspoint (i.e. in the period in which the timing signal SW is at highlevel), sample values included in the sample value series RS_(C) arecorrected to the distortion correction value amd detected on thedistortion correction value detection circuit 182. As a result, thewaveform distortion is corrected.

An effect obtained by correcting the waveform distortion will bedescribed with reference to FIG. 10 to FIG. 12. FIG. 10 is a waveformchart conceptually showing a waveform or the like of the read signalR_(RF) before and after the correction of the waveform distortion. FIG.11 are waveform charts conceptually showing the operation of obtainingthe high-frequency emphasized read sample value series RS_(H), on thesample value series RS_(C), in each of a case where the waveformdistortion is not corrected and a case where the waveform distortion iscorrected. FIG. 12 is a graph showing a change in symbol error rate withrespect to the waveform distortion ratio.

As shown on the left side of FIG. 10, if the waveform distortion occursin the read signal R_(RF), the waveform distortion is likely misjudgedto be the normal mark (e.g. the mark with a relatively short runlength). Therefore, the binary waveform after binarizing the read signalR_(RF) includes an error signal caused by the waveform distortion. Thisresults in inconsistency with the original record data and causes abinary error.

On the other hand, as shown on the right side of FIG. 10, if thewaveform distortion occurring in the read signal R_(RF) is corrected,the binary waveform after binarizing the read signal R_(RF) no longerincludes the error signal caused by the waveform distortion. Thisresults in consistency with the original record data and does not causethe binary error.

More specifically explaining this, depending on a condition such as themagnitude of the waveform distortion, as shown in FIG. 11( a), thewaveform distortion likely has a signal level which is greater than thelower limit −L of the amplitude limit value on the limit equalizer 15.In this case, the high-frequency emphasized read sample value seriesRS_(H) outputted from the high-frequency emphasis block 153 is the sumof a high-frequency emphasized read sample value series RS_(HIG) andS(0), and as described above,RS_(HIG)=(−k)×Sip(−1)+k×Sip(0)+k×Sip(1)+(−k)×Sip(2). Here, since Sip(−1)and Sip(2) are limited by the lower limit −L,RS_(H)=S(0)+k×(−2×−L+Sip(0)+Sip(1)). This increases the value of thehigh-frequency emphasized read sample value series RS_(H), by the valueobtained by multiplying the sum of the lower limit −L, Sip (0), andSip(1) by K. This is not preferable because it emphasizes the waveformdistortion which is originally not to occur. Moreover, for example, theemphasized waveform distortion likely leads to such a disadvantage thatthe mark with a relatively long run length in which the waveformdistortion occurs is misjudged to be another mark in an informationreproducing apparatus which adopts the PRML. This results in the binaryerror.

Moreover, this is not illustrated, but in the same manner, in theoptical disc 100 in which the reflectance of the laser beam LB isreduced by forming the marks shown in FIG. 6( a) to FIG. 6( c), Sip(−1)and Sip(2) are limited by the upper limit L, so thatRS_(H)=S(0)+k×(−2×L+Sip(0)+Sip(1)). This increases the value of thehigh-frequency emphasized read sample value series RS_(H), by the valueobtained by multiplying the sum of the upper limit L, Sip (0), andSip(1) by K. This is not preferable because it emphasizes the waveformdistortion which is originally not to occur.

On the other hand, as shown in FIG. 11( b), if the waveform distortionis corrected, the signal level of the waveform distortion can becorrected to be a signal level which is less than or equal to the lowerlimit −L of the amplitude limit value on the limit equalizer 15. In thiscase, Sip(−1) and Sip(0), and Sip(1) and Sip(2) are limited by the lowerlimit −L, so that RS_(H)=S(0). Thus, it is possible to prevent such adisadvantage that the waveform distortion is emphasized, and as aresult, it is possible to prevent such a disadvantage that the binaryerror occurs.

Moreover, this is not illustrated, but in the same manner, in theoptical disc 100 in which the reflectance of the laser beam LB isreduced by forming the marks shown in FIG. 6( a) to FIG. 6( c), if thewaveform distortion is corrected, Sip(−1) and Sip(0), and Sip(1) andSip(2) are limited by the upper limit L, so that RS_(H)=S(0). Thus, itis possible to prevent such a disadvantage that the waveform distortionis emphasized, and as a result, it is possible to prevent such adisadvantage that the binary error occurs.

As described above, the effect by correcting the waveform distortion canbe also seen from a change in symbol error rate with respect to thewaveform distortion ratio. As shown in FIG. 12, the value of SER in thecase where the waveform distortion is corrected is improved, compared tothe value of SER in the case where the waveform distortion is notcorrected.

As explained above, according to the information reproducing apparatus 1in the example, the dispersion of the sample values before and after thereference sample point is forcibly controlled in the read signal whichcauses the intersymbol interference in the high-frequency emphasis.Thus, even if the sufficient high-frequency emphasis is performed on thehigh-frequency emphasis block 153, the intersymbol interference does notoccur.

In particular, according to the information reproducing apparatus 1 inthe example, the amplitude limit and the high-frequency emphasis areperformed on the limit equalizer 15 after the waveform distortion iscorrected. Thus, it is possible to preferably prevent such adisadvantage that the waveform distortion which is originally not tooccur is emphasized on the limit equalizer 15. Moreover, it is possibleto preferably prevent such a disadvantage that the mark with arelatively long run length is misjudged to be another mark, in theinformation reproducing apparatus which adopts the PRML for example dueto the emphasized waveform distortion. As a result, the waveformdistortion rarely causes the binary error, and this allows thepreferable reproduction operation.

In addition, since the offset value OFS is added to the read signalR_(RF) and more specifically the read sample value series RS_(C)) beforethe waveform distortion is corrected, and the added offset value OFS issubtracted after the waveform distortion is corrected, the followingeffect can be also received. Hereinafter, with reference to FIG. 13 toFIG. 15, an explanation will be given on the effect obtained by addingand subtracting the offset value OFS. FIG. 13 are graphs showing achange in symbol error rate with respect to the offset value OFSnormalized by the amplitude of the read signal R_(RF), in each of a casewhere the offset value OFS is only added (i.e. where the offset valueOFS is not subtracted) and a case where the offset value OFS is addedand subtracted. FIG. 14 is a waveform chart conceptually showing awaveform of the read signal R_(RF) corresponding to minT according to achange in asymmetry. FIG. 15 is a waveform chart conceptually showinganother waveform or the like of the read signal R_(RF) before and afterthe correction of the waveform distortion.

As shown in FIG. 13, it can be seen that a range of the offset valueOFS, which improves the symbol error rate in the case where the offsetvalue OFS is added and subtracted, is expanded, compared to a range ofthe offset value OFS which improves the symbol error rate in the casewhere the offset value OFS is only added. In other words, by adding andsubtracting the offset value OFS, it is possible to expand the range ofthe offset value OFS which can preferably improves the symbol errorrate.

Moreover, it can be also seen from FIG. 13 that the symbol error rate isimproved by correcting the waveform distortion, as shown in FIG. 12described above.

As described above, the improvement of the reproduction properties (e.g.the symbol error rate) by adding (and further subtracting) the offsetvalue OFS can be explained from the following reasons.

As shown on the upper side of FIG. 14, if it is aimed at the opticaldisc 100 in which the reflectance of the laser beam LB is reduced byforming the marks, the signal level of the minT space is greater thanthe signal level of the minT mark. In this case, as the asymmetryincreases, the signal waveform of minT is gradually shifted to the lowerside (i.e. negative side), with respect to an all T center level (i.e.the reference level or the zero level). If the asymmetry increases tosome degree, the signal level of the peak of the minT space likely fallsbelow the all T center level. In this case, the minT is likely misjudgedto be the waveform distortion. As a result, as shown in FIG. 15, sincethe minT is corrected as the waveform distortion, a signal correspondingto the minT does not appear in the binary signal, which leads to thedeterioration of the symbol error rate.

In the same manner, as shown on the lower side of FIG. 14, if it isaimed at the optical disc 100 in which the reflectance of the laser beamLB is increased by forming the marks, the signal level of the minT spaceis less than the signal level of the minT mark. In this case, as theasymmetry increases, the signal waveform of minT is gradually shifted tothe upper side (i.e. positive side), with respect to the all T centerlevel (i.e. the reference level or the zero level). If the asymmetryincreases to some degree, the signal level of the peak of the minT spacelikely exceeds the all T center level. In this case, the minT is likelymisjudged to be the waveform distortion. As a result, since the minT iscorrected as the waveform distortion, the signal corresponding to theminT does not appear in the binary signal, which leads to thedeterioration of the symbol error rate.

However, according to the example, the signal waveform of the minT canbe shifted by adding the offset value OFS. As a result, it is possibleto preferably prevent such a disadvantage that the signal level of thepeak of the minT space described above falls below or exceeds the all Tcenter level (the reference level). As a result, it is possible topreferably prevent the deterioration of the symbol error rate.

Moreover, since the same offset value OFS is subtracted after thecorrection of the waveform distortion, it is also possible to receivesuch an effect that the offset value OFS can be relatively easilygenerated.

Incidentally, in the aforementioned example, the offset value OFS addedbefore the correction of the waveform distortion is same as the offsetvalue OFS subtracted after the correction of the waveform distortion;however, the offset value OFS added before the correction of thewaveform distortion is not necessarily same as the offset value OFSsubtracted after the correction of the waveform distortion. Hereinafter,with reference to FIG. 16 to FIG. 26, an explanation will be given on acase where the offset value OFS added before the correction of thewaveform distortion is different from the offset value OFS subtractedafter the correction of the waveform distortion.

Firstly, an explanation will be given on an example which enables theoffset value OFS added before the correction of the waveform distortionto be different from the offset value OFS subtracted after thecorrection of the waveform distortion on the basis of an asymmetryvalue, with reference to FIG. 16 and FIG. 17. FIG. 16 is a waveformchart conceptually showing an asymmetry value. FIG. 17 are tablesshowing the appearance probability of the record data with each runlength.

As shown in FIG. 16, the asymmetry value indicates the deviation of theamplitude center of the read signal corresponding to the record datawith the shortest run length, with respect to the amplitude center ofthe read signal R_(RF) corresponding to the record data with the longestrun length. Specifically, the asymmetry valueAsy=((ImaxH+ImaxL)−(IminH+IminL))/(2×(ImaxH−ImaxL)), wherein theamplitude center of the read signal R_(RF) corresponding to the datawith the longest run length is ImaxCnt, ImaxH is the magnitude of thetop amplitude of the read signal R_(RF) corresponding to the data withthe longest run length based on ImaxCnt, ImaxL is the magnitude of thebottom amplitude of the read signal R_(RF) corresponding to the datawith the longest run length based on ImaxCnt, IminH is the magnitude ofthe top amplitude of the read signal R_(RF) corresponding to the datawith the shortest run length based on ImaxCnt, and IminL is themagnitude of the bottom amplitude of the read signal R_(RF)corresponding to the data with the shortest run length based on ImaxCnt.Incidentally, ImaxCnt is an average value of the top amplitude value andthe bottom amplitude value of the read signal R_(RF) corresponding tothe data with the longest run length.

FIG. 17( a) shows the appearance probability, which considers the runlength, of the record data with each run length in 1ECC block if randomdata is recorded onto a Blu-ray Disc, which is one specific example ofthe optical disc 100. As shown in FIG. 17( a), in 1ECC block, theappearance probability of the record data with a run length of 2 T isabout 38%, the appearance probability of the record data with a runlength of 3 T is about 25%, the appearance probability of the recorddata with a run length of 4 T is about 16%, the appearance probabilityof the record data with a run length of 5 T is about 10%, the appearanceprobability of the record data with a run length of 6 T is about 6%, theappearance probability of the record data with a run length of 7 T isabout 3%, the appearance probability of the record data with a runlength of 8 T is about 1.6%, and the appearance probability of therecord data with a run length of 9 T is about 0.35%.

Incidentally, the appearance probability shown here (the T appearanceprobability in FIG. 17) is the appearance probability which does notconsider the run length. In other words, weighting is the same, which isused to calculate the appearance probability in each of the record datawith a run length of 2 T, the record data with a run length of 3 T, therecord data with a run length of 4 T, the record data with a run lengthof 5 T, the record data with a run length of 6 T, the record data with arun length of 7 T, the record data with a run length of 8 T, and therecord data with a run length of 9 T. In other words, it indicates theappearance probability in the case where the number of appearances iscounted as one if one record data with a certain run length appears.

In the example, the offset value OFS added before the correction of thewaveform distortion may be greater than the offset value OFS subtractedafter the correction of the waveform distortion, approximately by avalue which is obtained by multiplying the appearance probability, whichdoes not consider the run length, of the record data with the shortestrun length by the asymmetry value. In other words, in a Blu-ray Disc,which is one specific example of the optical disc 100, the offset valueOFS added before the correction of the waveform distortion may begreater than the offset value OFS subtracted after the correction of thewaveform distortion by 0.3809× the asymmetry value.

Moreover, FIG. 17( b) shows the appearance probability, which does notconsider the run length, of the record data with each run length in 1ECCblock if the random data is recorded onto a DVD, which is one specificexample of the optical disc 100. As shown in FIG. 17( b), in 1ECC block,the appearance probability of the record data with a run length of 3 Tis about 32%, the appearance probability of the record data with a runlength of 4 T is about 24%, the appearance probability of the recorddata with a run length of 5 T is about 17%, the appearance probabilityof the record data with a run length of 6 T is about 11.5%, theappearance probability of the record data with a run length of 7 T isabout 7%, the appearance probability of the record data with a runlength of 8 T is about 4%, the appearance probability of the record datawith a run length of 9 T is about 2%, the appearance probability of therecord data with a run length of 10 T is about 1.3%, the appearanceprobability of the record data with a run length of 11 T is about 0.24%,and the appearance probability of the record data with a run length of14 T is about 0.3%. Even in this case, the offset value OFS added beforethe correction of the waveform distortion may be greater than the offsetvalue OFS subtracted after the correction of the waveform distortion,approximately by the value which is obtained by multiplying theappearance probability, which does not consider the run length, of therecord data with the shortest run length, by the asymmetry value. Inother words, in a DVD, which is one specific example of the optical disc100, the offset value OFS added before the correction of the waveformdistortion may be greater than the offset value OFS subtracted after thecorrection of the waveform distortion by 0.3184× the asymmetry value.

Of course, in optical discs other than a Blu-ray Disc and a DVD, in thesame manner, the offset value OFS added before the correction of thewaveform distortion may be greater than the offset value OFS subtractedafter the correction of the waveform distortion, approximately by theappearance probability, which does not consider the run length, of therecord data with the shortest run length×the asymmetry value.

Next, an explanation will be given on an example which enables theoffset value OFS added before the correction of the waveform distortionto be different from the offset value OFS subtracted after thecorrection of the waveform distortion on the basis of an entire β value,with reference to FIG. 18. FIG. 18 is a waveform chart conceptuallyshowing the entire β value.

As shown in FIG. 18, the entire β value indicates the average positionof the amplitude center of the read signals R_(RF) corresponding to therespective record data with all types of run lengths (e.g. the recorddata with each of run lengths of 3 T to 11 T and 14 T if the opticaldisc 100 is a DVD, and the record data with each of run lengths of 2 Tto 9 T if the optical disc 100 is a Blu-ray Disc). Specifically, entireβ value=(A1+A2)/(A1−A2), wherein A1 is the magnitude of the maximumamplitude (top amplitude) on the upper side (positive side) which isbased on the amplitude center (i.e. all T center level) of the readsignals R_(RF) corresponding to the record data with all types of runlengths (i.e. the amplitude center is set at the origin or base point)and A2 is the magnitude of the maximum amplitude (bottom amplitude) onthe lower side (negative side) which is based on the amplitude center ofthe read signals R_(RF) corresponding to the record data with all typesof run lengths.

In the example, the offset value OFS added before the correction of thewaveform distortion may be greater than the offset value OFS subtractedafter the correction of the waveform distortion, approximately by avalue which is obtained by multiplying the appearance probability, whichdoes not consider the run length, of the record data with the shortestrun length by the entire β value. In other words, in a Blu-ray Disc,which is one specific example of the optical disc 100, the offset valueOFS added before the correction of the waveform distortion may begreater than the offset value OFS subtracted after the correction of thewaveform distortion by 0.3809× the entire β value.

In the same manner, in a DVD, which is one specific example of theoptical disc 100, the offset value OFS added before the correction ofthe waveform distortion may be greater than the offset value OFSsubtracted after the correction of the waveform distortion by 0.3184×the entire β value.

Of course, in optical discs other than a Blu-ray Disc and a DVD, in thesame manner, the offset value OFS added before the correction of thewaveform distortion may be greater than the offset value OFS subtractedafter the correction of the waveform distortion, approximately by theappearance probability, which does not consider the run length, of therecord data with the shortest run length×the entire value.

Next, an explanation will be given on an example which enables theoffset value OFS added before the correction of the waveform distortionto be different from the offset value OFS subtracted after thecorrection of the waveform distortion on the basis of a partial β value,with reference to FIG. 19. FIG. 19 is a waveform chart conceptuallyshowing the partial value.

As shown in FIG. 19, the partial β value indicates the deviation betweenthe amplitude center of the read signal R_(RF) corresponding to therecord data with the shortest run length and the amplitude center of theread signal R_(RF) corresponding to the record data with the secondshortest run length. Specifically, the partial β value=(Imin+1H+Imin+1L)/(Imin+1 H−Imin+1L), wherein the amplitude center of the readsignal corresponding to the record data with the shortest run length isIminCnt, Imin+1 H indicates the magnitude of the top amplitude of theread signal R_(RF) corresponding to the record data with the secondshortest run length based on IminCnt, and Imin+1L indicates themagnitude of the bottom amplitude of the read signal R_(RF)corresponding to the record data with the second shortest run lengthbased on IminCnt. Incidentally, IminCnt is an average value of the topamplitude value IminH and the bottom amplitude value IminL of the readsignal R_(RF) corresponding to the record data with the shortest runlength.

FIG. 17( a) shows the appearance probability, which considers the runlength, of the record data with each run length in 1ECC block if randomdata is recorded onto a Blu-ray Disc, which is one specific example ofthe optical disc 100. As shown in FIG. 17( a), in 1ECC block, theappearance probability of the record data with a run length of 2 T isabout 22%, the appearance probability of the record data with a runlength of 3 T is about 22%, the appearance probability of the recorddata with a run length of 4 T is about 19%, the appearance probabilityof the record data with a run length of 5 T is about 14%, the appearanceprobability of the record data with a run length of 6 T is about 10%,the appearance probability of the record data with a run length of 7 Tis about 6%, the appearance probability of the record data with a runlength of 8 T is about 4%, and the appearance probability of the recorddata with a run length of 9 T is about 0.9%.

Incidentally, the appearance probability shown here (the sampleappearance probability in FIG. 17) is the appearance probability, whichconsiders the run length. In other words, weighting is in proportion tothe run length, which is used to calculate the appearance probability ineach of the record data with a run length of 2 T, the record data with arun length of 3 T, the record data with a run length of 4 T, the recorddata with a run length of 5 T, the record data with a run length of 6 T,the record data with a run length of 7 T, the record data with a runlength of 8 T, and the record data with a run length of 9 T. In otherwords, it indicates the appearance probability in the case where thenumber of appearances is counted as n if one record data with a runlength of nT appears (i.e. if one record data including n sample valuesby sampling appears).

In the example, the offset value OFS added before the correction of thewaveform distortion may be greater than the offset value OFS subtractedafter the correction of the waveform distortion, approximately by avalue which is obtained by multiplying the appearance probability, whichdoes not consider the run length, of the record data with the shortestrun length by the partial β value. In other words, in a Blu-ray Disc,which is one specific example of the optical disc 100, the offset valueOFS added before the correction of the waveform distortion may begreater than the offset value OFS subtracted after the correction of thewaveform distortion by 0.2255× the partial β value.

Moreover, FIG. 17( b) shows the appearance probability, which considersthe run length, of the record data with each run length in 1ECC block ifthe random data is recorded onto a DVD, which is one specific example ofthe optical disc 100. As shown in FIG. 17( b), in 1ECC block, theappearance probability of the record data with a run length of 3 T isabout 20%, the appearance probability of the record data with a runlength of 4 T is about 20%, the appearance probability of the recorddata with a run length of 5 T is about 18%, the appearance probabilityof the record data with a run length of 6 T is about 14%, the appearanceprobability of the record data with a run length of 7 T is about 10%,the appearance probability of the record data with a run length of 8 Tis about 7%, the appearance probability of the record data with a runlength of 9 T is about 4.5%, the appearance probability of the recorddata with a run length of 10 T is about 3%, the appearance probabilityof the record data with a run length of 11 T is about 0.5%, and theappearance probability of the record data with a run length of 14 T isabout 0.9%. Even in this case, the offset value OFS added before thecorrection of the waveform distortion may be greater than the offsetvalue OFS subtracted after the correction of the waveform distortion,approximately by a value which is obtained by multiplying the appearanceprobability, which considers the run length, of the record data with theshortest run length by the partial β value. In other words, in a DVD,which is one specific example of the optical disc 100, the offset valueOFS added before the correction of the waveform distortion may begreater than the offset value OFS subtracted after the correction of thewaveform distortion by 0.2026× the partial β value.

Of course, in optical discs other than a Blu-ray Disc and a DVD, in thesame manner, the offset value OFS added before the correction of thewaveform distortion may be greater than the offset value OFS subtractedafter the correction of the waveform distortion, approximately by theappearance probability, which considers the run length, of the recorddata with the shortest run length× the partial β value.

Next, an explanation will be given on an example which enables theoffset value OFS added before the correction of the waveform distortionto be different from the offset value (WS subtracted after thecorrection of the waveform distortion on the basis of an α value, withreference to FIG. 20. FIG. 20 is a waveform chart conceptually showingthe α value.

As shown in FIG. 20, the α value indicates a deviation ratio (or rate)of the amplitude center of the read signal R_(RF) corresponding to therecord data with the shortest run length, with respect to the amplitudecenter (i.e. the reference level, and the zero level in the example) ofthe read signals R_(RF) corresponding to the respective record data withall types of run lengths (e.g. the record data with each of run lengthsof 3 T to 11 T and 14 T if the optical disc 100 is a DVD, and the recorddata with each of run lengths of 2 T to 9 T if the optical disc 100 is aBlu-ray Disc). Specifically, α value=ΔRef/(ImaxH−ImaxL), wherein ImaxHis the magnitude of the top amplitude of the read signal R_(RF)corresponding to the data with the longest run length based on theamplitude center of the read signal R_(RF) corresponding to the recorddata with all types of run lengths (i.e. all T center level), ImaxL isthe magnitude of the bottom amplitude of the read signal R_(RF)corresponding to the data with the longest run length based on theamplitude center of the read signals R_(RF) corresponding to the recorddata with all types of run lengths (i.e. all T center level), and ΔRefis a shift amount of the amplitude center of the read signal R_(RF)corresponding to the record data with the shortest run length, withrespect to the amplitude center of the read signals R_(RF) correspondingto the record data with all types of run lengths.

In the example, the offset value OFS added before the correction of thewaveform distortion may be greater than the offset value OFS subtractedafter the correction of the waveform distortion, approximately by the avalue.

As described above, by increasing the offset value OFS added before thecorrection of the waveform distortion, by a magnitude corresponding tothe value determined in accordance with the asymmetry value, the entireβ value, or the partial β value, compared to the offset value OFSsubtracted after the correction of the waveform distortion, it ispossible to leave an offset component with the aforementioned magnitudecorresponding to the value determined in accordance with the asymmetryvalue, the entire β value, or the partial β value, in thedistortion-corrected read sample value series RS_(CAM) after thecorrection of the waveform distortion. This allows the amplitude centerof the read signal R_(RF) corresponding to the record data with theshortest run length to be brought closer to the reference level, in thedistortion-corrected read sample value series RS_(CAM) after thecorrection of the waveform distortion.

Moreover, by increasing the offset value OFS by the magnitude (the αvalue) corresponding to the deviation between the reference level andthe amplitude center of the read signal R_(RF) corresponding to therecord data with the shortest run length, it is possible to match thereference level and the amplitude center of the read signal R_(RF)corresponding to the record data with the shortest run length.

Incidentally, in the operation example shown in FIG. 7, only onewaveform distortion correction condition is set; however, a plurality ofwaveform distortion correction conditions may be set, and the waveformdistortion correction may be performed by applying the waveformdistortion correction conditions in order. The operation example in thiscase will be described with reference to FIG. 21. FIG. 21 is a flowchartconceptually showing another flow of operations of the adder 19-1, theoffset addition circuit 19-2, the subtracter 19-3, and the waveformdistortion correction circuit 18.

As shown in FIG. 21, firstly, the operation of reproducing data recordedon the optical disc 100 is performed (step S101). In the reproductionoperation, it is sequentially judged whether or not a symbol error rateis greater than or equal to a predetermined threshold value, or whetheror not error correction is unable to be performed, or whether or not thesynchronization data is unable to be read (step S102).

As a result of the judgment in the step S102, if it is judged that thesymbol error rate is not greater than or equal to the threshold value,and that the error correction is not unable to be performed, and thatthe synchronization data is not unable to be read (the step S102: No),the operational flow goes to a step S112.

On the other hand, as a result of the judgment in the step S102, if itis judged that the symbol error rate is greater than or equal to thethreshold value, or that the error correction is unable to be performed,or that the synchronization data is unable to be read (the step S102:Yes), then, the waveform distortion of the long mark is measured (stepS103). Then, it is judged whether or not the waveform distortion isgreater than or equal to a predetermined value (step S104).

As a result of the judgment in the step S104, if it is judged that thewaveform distortion is not greater than or equal to the predeterminedvalue (e.g. that the waveform distortion ratio is less than or equal toapproximately 30%) (the step S104: No), the operational flow goes to thestep S112.

On the other hand, as a result of the judgment in the step S104, if itis judged that the waveform distortion is greater than or equal to thepredetermined value (e.g. that the waveform distortion ratio is greaterthan or equal to approximately 30%) (the step S104: Yes), then by theoperation of the offset generation circuit 19-2, it is judged whether ornot the offset value OFS is to be added to the read signal R_(RF) (andmore specifically the read sample value series RS_(C)) (step S105).

As a result of the judgment in the step S105, if it is judged that theoffset value OFS is not to be added (the step S105: No), the offsetvalue OFS is set to zero (step S107), and then the operational flow goesto a step S108.

On the other hand, as a result of the judgment in the step S105, if itis judged that the offset value OFS is to be added (the step S105: Yes),the offset value OFS is generated by the operation of the offsetgeneration circuit 19-2 (step S106), and then the operational flow goesto the step S108.

Then, the offset value OFS, which is generated by the operation of theadder 19-1 in the step S106 or S107, is added to the read signal R_(RF)(and more specifically, the read sample value series RS_(C)) (stepS108).

Then, a waveform distortion correction condition #x (wherein x is aninteger of 1 or more, with an initial value of 1), such as a correctionlevel and a correction range for the waveform distortion, is set (stepS201). Then, on the basis of the waveform distortion correctioncondition #x set in the step S201, the waveform distortion of the longmark is corrected (step S110).

Then, as a result of the correction based on the waveform distortioncorrection condition #x, it is judged whether or not a target conditionis realized (step S202). As the target condition, for example, thejudgment condition in the step S102 (i.e. that the symbol error rate isgreater than or equal to the predetermined threshold value, or that theerror correction is unable to be performed) may be used.

As a result of the judgment in the step S202, if it is judged that thetarget condition is realized (the step S202: Yes), the offset value OFSadded in the step S108 is subtracted from the distortion-corrected readsample value series RS_(CAM) (step S111); namely, the same offset valueas that added in the step S108 is subtracted from thedistortion-corrected read sample value series RS_(CAM). Then, theoperational flow goes to the step S112.

On the other hand, as a result of the judgment in the step S202, if itis judged that the target condition is not realized (the step S202: No),x is incremented by 1 (step S203), and then, the operations after thestep S201 are repeated. In other words, until the target condition isrealized, the waveform distortion is corrected while the waveformdistortion correction condition is changed as occasion demands.

Incidentally, as the plurality of waveform distortion correctionconditions, it is preferable to use the waveform distortion correctionconditions used in the operations in modified examples detailed laterwith reference to FIG. 23 to FIG. 36.

Moreover, in the aforementioned example, such construction is adoptedthat the addition of the offset value OFS is performed once perreproduction; however, the offset value may be added a plurality oftimes per reproduction in a stepwise manner. The operation example inthis case will be described with reference to FIG. 22. FIG. 22 is aflowchart conceptually showing another flow of operations of the adder194, the offset addition circuit 19-2, the subtracter 19-3, and thewaveform distortion correction circuit 18.

As shown in FIG. 22, firstly, the operation of reproducing data recordedon the optical disc 100 is performed (step S101). Here, a variable nused in the addition of the offset value OFS is set to an initial value0 (step S401). In the reproduction operation, it is sequentially judgedwhether or not a symbol error rate is greater than or equal to apredetermined threshold value, or whether or not error correction isunable to be performed, or whether or not the synchronization data isunable to be read (step S102).

As a result of the judgment in the step S102, if it is judged that thesymbol error rate is not greater than or equal to the threshold value,and that the error correction is not unable to be performed, and thatthe synchronization data is not unable to be read (the step S102: No),the operational flow goes to a step S112.

On the other hand, as a result of the judgment in the step S102, if itis judged that the symbol error rate is greater than or equal to thethreshold value, or that the error correction is unable to be performed,or that the synchronization data is unable to be read (the step S102:Yes), then, the waveform distortion of the long mark is measured (stepS103). Then, it is judged whether or not the waveform distortion isgreater than or equal to a predetermined value (step S104).

As a result of the judgment in the step S104, if it is judged that thewaveform distortion is not greater than or equal to the predeterminedvalue (e.g. that the waveform distortion ratio is less than or equal toapproximately 30%) (the step S104: No), then, it is judged whether ornot the retry number is greater than or equal to a predetermined value,wherein the retry number is the number of times to add the offset valueOFS (step S404).

As a result of the judgment in the step S404, if it is judged that theretry number is not greater than or equal to the predetermined value(the step S404: No), the operational flow returns to the step S102, andthe operations after the step S102 are repeated.

On the other hand, as a result of the judgment in the step S404, if itis judged that the retry number is greater than or equal to thepredetermined value (the step S404: Yes), the operational flow goes tothe step S112.

On the other hand, as a result of the judgment in the step S104, if itis judged that the waveform distortion is greater than or equal to thepredetermined value (e.g. that the waveform distortion ratio is greaterthan or equal to approximately 30%) (the step S104: Yes), then by theoperation of the offset generation circuit 19-2, it is judged whether ornot the offset value OFS is to be added to the read signal R_(RF) (andmore specifically the read sample value series RS_(C)) (step S105).

As a result of the judgment in the step S105, if it is judged that theoffset value OFS is not to be added (the step S105: No), the offsetvalue OFS is set to be zero (step S107), and then the operational flowgoes to a step S108.

On the other hand, as a result of the judgment in the step S105, if itis judged that the offset value OFS is to be added (the step S105: Yes),the variable n is incremented by 1 (step S402). Then, the offset valueOFS is generated by the operation of the offset generation circuit 19-2such that the offset value OFS normalized by the amplitude of the readsignal R_(RF) is n % (step S403). Then, the operational flow goes to thestep S108.

Then, the offset value OFS, which is generated by the operation of theadder 19-1 in the step S402 or S107, is added to the read signal R_(RF)(and more specifically the read sample value series RS_(C)) (step S108).

Then, a waveform distortion correction condition, such as a correctionlevel and a correction range for the waveform distortion, is set (stepS109). Then, the waveform distortion of the long mark is corrected onthe basis of the waveform distortion correction condition set in thestep S109 (step S110).

Then, the offset value OFS added in the step S108 is subtracted from thedistortion-corrected read sample value series RS_(CAM) (step S111);namely, the same offset value as that added in the step S108 issubtracted from the distortion-corrected read sample value seriesRS_(CAM).

Then, it is judged whether or not the reproduction operation is to beended (step S112), and if the reproduction operation is not to be ended(the step S112: No), the operational flow returns to the step S101, andthe operations after the step S101 are repeated again.

(4) Modified Examples

Next, with reference to FIG. 23 to FIG. 36, an explanation will be givenon modified examples of the information reproducing apparatus 1 in theexample.

(4-1) First Modified Example

Firstly, with reference to FIG. 23 and FIG. 24, an informationreproducing apparatus 1 a in a first modified example will be described.FIG. 23 is a waveform chart conceptually showing an operation ofcorrecting the waveform distortion by a waveform distortion correctioncircuit 18 a provided for the information reproducing apparatus 1 a inthe first modified example, on the sample value series RS_(C). FIG. 24is a block diagram conceptually showing the structure of the waveformdistortion correction circuit 18 a provided for the informationreproducing apparatus 1 a in the first modified example.

Incidentally, the same structures and operations as those in theaforementioned example carry the same reference numerals, and theexplanation thereof is omitted.

As shown in FIG. 23, in the first modified example, as the distortioncorrection value amd, the average value of center samples of a mark witha run length of (min+3) T (i.e. the minimum amplitude value of the markwith (min+3) T for the waveform distortion shown in FIG. 5( a) to FIG.5( c), and the maximum amplitude value of the mark with (min+3) T forthe waveform distortion shown in FIG. 6( a) to FIG. 6( c)) is used.

Incidentally, (min+k)T indicates the read signal R_(RF) (and morespecifically, the read sample value series RS_(C) corresponding to theread signal R_(RF)) corresponding to the record data with the (k+1)thshortest run length (wherein k is an integer or 1 or more). Therefore,(min+3) T indicates the read signal R_(RF) (and more specifically, theread sample value series RS_(C) corresponding to the read signal R_(RF))corresponding to the record data with the fourth shortest run length.For example, if the optical disc 100 is a DVD, (min+3) T indicates theread signal R_(RF) corresponding to the record data with a run length of6 T. For example, if the optical disc 100 is a Blu-ray Disc, (min+3) Tindicates the read signal R_(RF) corresponding to the record data with arun length of 5 T.

In this case, as shown in FIG. 24, the waveform distortion correctioncircuit 18 a is provided with the delay adjustment circuit 181, adistortion correction value detection circuit 182 a, the mark/spacelength detection circuit 183, the timing generation circuit 184, and theselector 185.

The distortion correction value detection circuit 182 a holds andaverages the center sample values of the record data if the record datawith a run length of (min+3) T is inputted, while monitoring themark/space length outputted from the mark/space length detection circuit183, and outputs it to the selector 185 as the distortion correctionvalue.

As described above, even if the average value of the center samples ofthe record data with a run length of (min+3) T is used as the distortioncorrection value amd, it is possible to preferably receive theaforementioned various effects.

Moreover, depending on circumstances, by correcting the waveformdistortion, the signal level after the correction is increased from theoriginal signal level (i.e. the signal level before the correction).Thus, by correcting the waveform distortion, it is possible to bring thesignal level closer to the maximum amplitude of the read signal R_(RF).As a result, particularly in the information reproducing apparatus whichadopts the PRML (Partial Response Maximum Likelihood), the record datacan be reproduced more preferably.

Incidentally, as the distortion correction value amd, instead of theaverage value of the center samples of the record data with a run lengthof (min+3) T, the average value of the center samples of the record datawith another run length may be used. In this case, as the record datawith another run length, the record data which can realize the maximumamplitude is preferably used.

(4-2) Second Modified Example

Next, with reference to FIG. 25 and FIG. 26, an information reproducingapparatus 1 b in a second modified example will be described. FIG. 25 isa waveform chart conceptually showing an operation of correcting thewaveform distortion by a waveform distortion correction circuit 18 bprovided for the information reproducing apparatus 1 b in the secondmodified example, on the sample value series RS_(C). FIG. 26 is a blockdiagram conceptually showing the structure of the waveform distortioncorrection circuit 18 b provided for the information reproducingapparatus 1 b in the second modified example.

Incidentally, the same structures and operations as those in theaforementioned example carry the same reference numerals, and theexplanation thereof is omitted.

As shown in FIG. 25, in the second modified example, as the distortioncorrection value amd, the maximum value or minimum value of a digitalcode for indicating the read sample value series RS_(H) (i.e. theminimum value of the digital code for the waveform distortion shown inFIG. 5( a) to FIG. 5( c), and the minimum value of the digital code forthe waveform distortion shown in FIG. 6( a) to FIG. 6( c)) is used. Forexample, if the digital code is 8-bit, the maximum value of the digitalcode is 2^(8−1)−1=127, and the minimum value of the digital code is−2^(8−1)−1=−128.

In this case, as shown in FIG. 26, the waveform distortion correctioncircuit 18 b is provided with the delay adjustment circuit 181, adistortion correction value detection circuit 182 b, the mark/spacelength detection circuit 183, the timing generation circuit 184, and theselector 185.

The distortion correction value detection circuit 182 b outputs themaximum value or minimum value of the digital code, to the selector 185as the distortion correction value.

As described above, even if the maximum value or minimum value of thedigital code is used as the distortion correction value amd, it ispossible to preferably receive the aforementioned various effects.

In addition, it is no longer necessary to sequentially detect thedistortion correction value amd, so that it is possible to relativelyreduce a load of the waveform distortion correction circuit 18 b (i.e. aload of the information reproducing apparatus 1 b).

Incidentally, if not only the maximum value or minimum value of thedigital code but also a predetermined fixed value is used as thedistortion correction value amd, it is possible to preferably receivethe aforementioned various effects while relatively reducing the load ofthe waveform distortion correction circuit 18 b (i.e. the load of theinformation reproducing apparatus 1 b).

(4-3) Third Modified Example

Next, with reference to FIG. 27 and FIG. 28, an information reproducingapparatus 1 c in a third modified example will be described. FIG. 27 isa waveform chart conceptually showing an operation of correcting thewaveform distortion by a waveform distortion correction circuit 18 cprovided for the information reproducing apparatus 1 c in the thirdmodified example, on the sample value series RS_(C). FIG. 28 is a blockdiagram conceptually showing the structure of the waveform distortioncorrection circuit 18 c provided for the information reproducingapparatus 1 c in the third modified example.

Incidentally, the same structures and operations as those in theaforementioned example carry the same reference numerals, and theexplanation thereof is omitted.

As shown in FIG. 27, in the third modified example, as the distortioncorrection value amd, the upper limit L or lower limit −L of theamplitude limit value on the limit equalizer 15 (i.e. the lower limit −Lof the amplitude limit value for the waveform distortion shown in FIG.5( a) to FIG. 5( c), and the upper limit L of the amplitude limit valuefor the waveform distortion shown in FIG. 6( a) to FIG. 6( e)) is used.

In this case, as shown in FIG. 28, the waveform distortion correctioncircuit 18 c is provided with the delay adjustment circuit 181, themark/space length detection circuit 183, the timing generation circuit184, and the selector 185.

The selector 185 outputs the upper limit L or lower limit −L of theamplitude limit value on the limit equalizer 15 as the distortioncorrection sample value series RS_(CAM) if the high-level timing signalSW is outputted from the timing generation circuit 184.

As described above, even if the upper limit L or lower limit −L of theamplitude limit value on the limit equalizer 15 is used as thedistortion correction value amd, it is possible to preferably receivethe aforementioned various effects.

In addition, since the signal level of the waveform distortion iscorrected to the upper limit L or lower limit −L of the amplitude limitvalue on the limit equalizer 15, it is certainly possible to preventsuch a disadvantage that the waveform distortion which is originally notto occur is emphasized. Moreover, it is also possible to preferablyprevent such a disadvantage that the mark with a relatively long runlength is misjudged to be another mark, caused by the emphasizedwaveform distortion, in the information reproducing apparatus whichadopts the PRML. As a result, the binary error caused by the waveformdistortion hardly occurs, which allows the preferable reproductionoperation.

Incidentally, a value which is greater than or equal to the upper limitL or a value which is less than or equal to the lower limit −L of theamplitude limit value on the limit equalizer 15 may be used. Even insuch construction, it is possible to preferably receive theaforementioned various effects.

(4-4) Fourth Modified Example

Next, with reference to FIG. 29 and FIG. 30, an information reproducingapparatus 1 d in a fourth modified example will be described. FIG. 29 isa waveform chart conceptually showing an operation of correcting thewaveform distortion by a waveform distortion correction circuit 18 dprovided for the information reproducing apparatus 1 d in the fourthmodified example, on the sample value series RS_(C). FIG. 30 is a blockdiagram conceptually showing the structure of the waveform distortioncorrection circuit 18 d provided for the information reproducingapparatus 1 d in the fourth modified example.

Incidentally, the same structures and operations as those in theaforementioned example carry the same reference numerals, and theexplanation thereof is omitted.

As shown in FIG. 29, in the fourth modified example, as the distortioncorrection value amd, a value (i.e. 2L or −2L) twice the upper limit Lor lower limit −L of the amplitude limit value on the limit equalizer 15(i.e. the lower limit L of the amplitude limit value for the waveformdistortion shown in FIG. 5( a) to FIG. 5( c), and the upper limit L ofthe amplitude limit value for the waveform distortion shown in FIG. 6(a) to FIG. 6( c)) is used.

In this case, as shown in FIG. 30, the waveform distortion correctioncircuit 18 d is provided with the delay adjustment circuit 181, anamplifier 182 d, the mark/space length detection circuit 183, the timinggeneration circuit 184, and the selector 185.

The amplifier 182 d amplifies the upper limit L or lower limit −L of theamplitude limit value on the limit equalizer 15 by two times, and thenoutputs it to the selector 185 as the distortion correction value amd.

As described above, even if the value which is twice the upper limit Lor lower limit −L of the amplitude limit value on the limit equalizer 15is used as the distortion correction value amd, it is possible topreferably receive the aforementioned various effects.

In addition, since the signal level of the waveform distortion iscorrected to the upper limit L or lower limit −L of the amplitude limitvalue on the limit equalizer 15, it is certainly possible to preventsuch a disadvantage that the waveform distortion which is originally notto occur is emphasized. Moreover, it is also possible to preferablyprevent such a disadvantage that the mark with a relatively long runlength is misjudged to be another mark, caused by the emphasizedwaveform distortion, in the information reproducing apparatus whichadopts the PRML. As a result, the binary error caused by the waveformdistortion hardly occurs, which allows the preferable reproductionoperation.

Moreover, even if a noise component is superimposed on the read signalR_(RF), since the signal level of the waveform distortion is correctedto the signal level which is less than or equal to the value which istwice the upper limit L or the lower limit −L of the amplitude limitvalue, it is certainly possible to prevent such a disadvantage that thewaveform distortion is less than or equal to the upper limit L of theamplitude limit value or is greater than or equal to the lower limit −L.As a result, it is possible to preferably prevent such a disadvantagethat the long mark is misjudged to be another mark. As a result, thebinary error caused by the waveform distortion hardly occurs, whichallows the preferable reproduction operation.

(4-5) Fifth Modified Example

Next, with reference to FIG. 31 to FIG. 34, an information reproducingapparatus 1 c in a fifth modified example will be described. FIG. 31 isa timing chart conceptually showing an operation of correcting thewaveform distortion by a waveform distortion correction circuit 18 eprovided for the information reproducing apparatus 1 c in the fifthmodified example, on a first read signal R_(RF). FIG. 32 is a timingchart conceptually showing the operation of correcting the waveformdistortion by the waveform distortion correction circuit 18 e providedfor the information reproducing apparatus 1 c in the fifth modifiedexample, on a second read signal R_(RF). FIG. 33 is a flowchartconceptually showing a first flow of operations by the waveformdistortion correction circuit 18 e provided for the informationreproducing apparatus 1 c in the fifth modified example. FIG. 34 is aflowchart conceptually showing a second flow of operations by thewaveform distortion correction circuit 18 e provided for the informationreproducing apparatus 1 c in the fifth modified example.

The record data recorded on the optical disc 100 includes not onlynormal user data but also the synchronization data (e.g. the record datawith a run length of 14 T if the optical disc 100 is a DVD, and therecord data with a run length of 9 T if the optical disc 100 is aBlu-ray Disc) used for synchronization in reproducing the user data.Considering that the synchronization data is included in the recorddata, the correction of the waveform distortion may be limited to thesynchronization data.

More specifically, as shown in FIG. 31, if the optical disc 100 is aBlu-ray Disc, since the synchronization data is formed of a 9 T mark anda 9 T space, firstly, the 9 T space is detected, and the waveformdistortion before or after the detected 9 T space may be corrected.Moreover, focusing on the periodicity that the synchronization dataappears, the waveform distortion may be corrected near a position beingshifted by a time corresponding to 1932 T (or 1932 T±α1: α1 is apredetermined constant) from the detected 9 T space toward the advancingdirection (or a position being shifted by β1 T from the relevantposition toward the advancing direction: β 1 is a predeterminedconstant).

Moreover, as shown in FIG. 32, if the optical disc 100 is a DVD, sincethe synchronization data is a 14 T mark or a 14 T space, firstly, the 14T space is detected, and the waveform distortion may be corrected near aposition being shifted by a time corresponding to 1488 T (or 1488 T±α2:α2 is a predetermined constant) from the detected 14 T space toward theadvancing direction (or a position being shifted by β2 T from therelevant position toward the advancing direction: β2 is a predeterminedconstant).

A flow of the operations in this case will be described. As shown inFIG. 33, firstly, the operation of reproducing data recorded on theoptical disc 100 is performed (step S101). In the reproductionoperation, it is judged whether or not a sync space (i.e. a space whichconstitutes the synchronization data: the aforementioned 9 T space, 14 Tspace, and the like) is detected (step S301).

As a result of the judgment in the step S301, if it is judged that thesync space is not detected (the step S301: No), the operational flowreturns to the step S301, and the operation of judging whether or notthe sync space is detected is repeated.

On the other hand, as a result of the judgment in the step S301, if itis judged that the sync space is detected (the step S301: Yes), then, itis judged whether or not the mark is reproduced at a position beingshifted by a time corresponding to nT from the sync space toward theadvancing direction (step S302). In other words, focusing on theperiodicity that the synchronization data appears, it is judged whetheror not the mark is reproduced at a position being shifted by theaforementioned time corresponding to 1932 T±α1 or 1488 T±α 2 from thedetected sync space toward the advancing direction.

As a result of the judgment in the step S302, if it is judged that themark is not reproduced at the position being shifted by the timecorresponding to nT from the sync space toward the advancing direction(the step S302: No), the operation in the step S302 is repeated.

On the other hand, as a result of the judgment in the step S302, if itis judged that the mark is reproduced at the position being shifted bythe time corresponding to nT from the sync space toward the advancingdirection (the step S302: Yes), then, the waveform distortion of themark corresponding to the synchronization data is measured near theposition being shifted by the time corresponding to nT from the syncspace toward the advancing direction (step S105). Subsequently, the sameoperations shown in FIG. 7 are performed.

Moreover, in this case, as shown in FIG. 34, as in the operation exampleshown in FIG. 21, a plurality of waveform distortion correctionconditions may be set, and the waveform distortion correction may beperformed by applying the waveform distortion correction conditions inorder.

Specifically, as shown in FIG. 34, firstly, the operation of reproducingdata recorded on the optical disc 100 is performed (step S101). Then, itis judged whether or not the sync space (i.e. the space whichconstitutes the synchronization data: the aforementioned 9 T space, 14 Tspace, and the like) is detected (step S301).

As a result of the judgment in the step S301, if it is judged that thesync space is not detected (the step S301: No), the operational flowreturns to the step S301, and the operation of judging whether or notthe sync space is detected is repeated.

On the other hand, as a result of the judgment in the step S301, if itis judged that the sync space is detected (the step S301: Yes), then, itis judged whether or not the mark is reproduced at a position beingshifted by a time corresponding to nT from the sync space toward theadvancing direction (step S302). In other words, focusing on theperiodicity that the synchronization data appears, it is judged whetheror not the mark is reproduced at a position being shifted by theaforementioned time corresponding to 1932 T±α1 or 1488 T±α2 from thedetected sync space toward the advancing direction.

As a result of the judgment in the step S302, if it is judged that themark is not reproduced at the position being shifted by the timecorresponding to nT from the sync space toward the advancing direction(the step S302: No), the operation in the step S302 is repeated.

On the other hand, as a result of the judgment in the step S302, if itis judged that the mark is reproduced at the position being shifted bythe time corresponding to nT from the sync space toward the advancingdirection (the step S302: Yes), then, the waveform distortion of themark corresponding to the synchronization data is measured near theposition being shifted by the time corresponding to nT from the syncspace toward the advancing direction (step S105). Subsequently, the sameoperations shown in FIG. 21 are performed.

As described above, by correcting the waveform distortion in view ofthat the synchronization data is included in the record data, it ispossible to preferably perform the high-frequency emphasis on thesynchronization data which is more important than the user data,resulting in the preferable reproduction of the synchronization data.This can further increase the stability of the reproduction operation.

(4-6) Sixth Modified Example

Next, with reference to FIG. 35 and FIG. 36, an information reproducingapparatus 1 f in a sixth modified example will be described. FIG. 35 isa block diagram conceptually showing the structure of a waveformdistortion correction circuit 18 f provided for the informationreproducing apparatus 1 f in a sixth modified example. FIG. 36 is ablock diagram conceptually showing the structure of a waveformdistortion detection circuit 186 f provided for the waveform distortioncorrection circuit 18 f provided for the information reproducingapparatus 1 f in the sixth modified example.

As shown in FIG. 35, the waveform distortion correction circuit 18 f isprovided with a delay adjustment circuit 181, the waveform distortiondetection circuit 186 f, the mark/space length detection circuit 183,the timing generation circuit 184, the selector 185, and an AND circuit187 f.

In this aspect, a result of the detection of the mark/space length bythe mark/space length detection circuit 183 is outputted to the waveformdistortion detection circuit 186 f, in addition to the timing generationcircuit 184.

The waveform distortion detection circuit 186 f detects the waveformdistortion and outputs a waveform distortion detection signal DT whichindicates that the waveform distortion is detected, to the AND circuit187 f. More specifically, the waveform distortion detection circuit 186f outputs a high-level waveform distortion detection signal DT (DT=1) tothe AND circuit 187 f if the waveform distortion is detected, andoutputs a low-level waveform distortion detection signal DT (DT=0) tothe AND circuit 187 f if the waveform distortion is not detected.

The AND circuit 187 f generates a high-level timing signal SW0 if thewaveform distortion is detected (if each of the timing signal SWoutputted from the timing generation circuit 184 and the waveformdistortion detection signal DT outputted from the waveform distortiondetection circuit 186 f is high-level), on the basis of the output ofeach of the timing generation circuit 184 and the waveform distortiondetection circuit 186 f. On the other hand, the AND circuit 187 fgenerates a low-level timing signal SW0 if the waveform distortion isnot detected (if either the timing signal SW outputted from the timinggeneration circuit 184 or the waveform distortion detection signal DToutputted from the waveform distortion detection circuit 186 f islow-level), on the basis of the output of each of the timing generationcircuit 184 and the waveform distortion detection circuit 186 f. Inother words, in the sixth modified example, the waveform distortion iscorrected selectively when the waveform distortion is detected.

As shown in FIG. 36, the waveform distortion detection circuit 186 f isprovided with a shift register 1831 f, a selector 1832 f, a maximumvalue detection circuit 1833 f, a minimum value detection circuit 1834f, a subtracter 1835 f, and a judgment circuit 1836 f.

The read sample value series RS_(C) inputted to the waveform distortiondetection circuit 186 f is outputted to the shift register 1831 f. Theshift register 1831 f outputs the inputted read sample value seriesRS_(C) to the selector 1832 f as outputs D0 to D14 while shifting theinputted read sample value series RS_(C) by one clock.

The selector 1832 f selectively samples and holds three outputs fromamong the outputs D0 to D14, on the basis of the mark/space length, intiming outputted from the mark/space length detection circuit 183, andoutputs the three outputs to a distortion correction amount detectioncircuit 1837 f, the maximum value detection circuit 1833 f, and theminimum value detection circuit 1834 f, respectively.

More specifically, the selector 1832 f selectively samples and holdsthree outputs D2, D3, and D4 from among the outputs D0 to D14 if themark/space length outputted from the mark/space length detection circuit183 is 6 T, and outputs the three outputs to the distortion correctionamount detection circuit 1837 f, the maximum value detection circuit1833 f, and the minimum value detection circuit 1834 f, respectively.The selector 1832 f selectively samples and holds three outputs D2, D3,and D5 from among the outputs D0 to D14 if the mark/space lengthoutputted from the mark/space length detection circuit 183 is 7 T, andoutputs the three outputs to the distortion correction detection circuit1837 f, the maximum value detection circuit 1833 f, and the minimumvalue detection circuit 1834 f, respectively. The selector 1832 fselectively samples and holds three outputs D2, D4, and D6 from amongthe outputs D0 to D14 if the mark/space length outputted from themark/space length detection circuit 183 is 8 T, and outputs the threeoutputs to the distortion correction amount detection circuit 1837 f,the maximum value detection circuit 1833 f, and the minimum valuedetection circuit 1834 f, respectively. The selector 1832 f selectivelysamples and holds three outputs D2, D4, and D7 from among the outputs D0to D14 if the mark/space length outputted from the mark/space lengthdetection circuit 183 is 9 T, and outputs the three outputs to thedistortion correction amount detection circuit 1837 f, the maximum valuedetection circuit 1833 f, and the minimum value detection circuit 1834f, respectively. The selector 1832 f selectively samples and holds threeoutputs D2, D5, and D8 from among the outputs D0 to D14 if themark/space length outputted from the mark/space length detection circuit183 is 10 T, and outputs the three outputs to the distortion amountcorrection detection circuit 1837 f, the maximum value detection circuit1833 f, and the minimum value detection circuit 1834 f, respectively.The selector 1832 f selectively samples and holds three outputs D2, D5,and D9 from among the outputs D0 to D14 if the mark/space lengthoutputted from the mark/space length detection circuit 183 is 11 T, andoutputs the three outputs to the distortion correction amount detectioncircuit 1837 f, the maximum value detection circuit 1833 f, and theminimum value detection circuit 1834 f, respectively. The selector 1832f selectively samples and holds three outputs D2, D7, and D12 from amongthe outputs D0 to D14 if the mark/space length outputted from themark/space length detection circuit 183 is 14 T, and outputs the threeoutputs to the distortion correction amount detection circuit 1837 f,the maximum value detection circuit 1833 f, and the minimum valuedetection circuit 1834 f, respectively. The operation of the selector1832 f described above substantially corresponds to the operation ofselectively outputting the signal level in the front edge portion, thesignal level in the middle portion, and the signal level in the rearedge portion of the waveform distortion, shown in FIG. 5( a) to FIG. 5(c) and FIG. 6( a) to FIG. 6( c).

Then, on the distortion correction amount detection circuit 1837 f,desired one signal level of the three outputs outputted from theselector 1832 f (i.e. the signal level in the front edge portion, thesignal level in the middle portion, and the signal level in the rearedge portion) is outputted as the distortion correction value amd.Specifically, as shown in FIG. 5( a) and FIG. 6( a), for the waveformdistortion in which the signal level in the middle portion is changed,for example, the signal level in the front edge portion or the signallevel in the rear edge portion is outputted as the distortion correctionvalue amd. As shown in FIG. 5( b) and FIG. 6( b), for the waveformdistortion in which the signal level in the front edge portion ischanged, for example, the signal level in the rear edge portion isoutputted as the distortion correction value amd. As shown in FIG. 5( c)and FIG. 6( c), for the waveform distortion in which the signal level inthe rear edge portion is changed, for example, the signal level in thefront edge portion is outputted as the distortion correction value amd.

Moreover, on the maximum value detection circuit 1833 f, the maximumvalue (i.e. the maximum signal level) of the three outputs outputtedfrom the selector 1832 f is detected, and the detected maximum value isoutputted to the subtracter 1835 f.

Moreover, on the minimum value detection circuit 1834 f, the minimumvalue (i.e. the minimum signal level) of the three outputs outputtedfrom the selector 1832 f is detected, and the detected minimum value isoutputted to the subtracter 1835 f.

Then, on the subtracter 1835 f, the minimum value detected on theminimum value detection circuit 1834 f is subtracted from the maximumvalue detected on the maximum value detection circuit 1833 f, by whichthe waveform distortion amount D is calculated.

Then, on the judgment circuit 1836 f, it is judged whether or not thewaveform distortion amount outputted from the subtracter 1835 f isgreater than or equal to a predetermined value x. If the waveformdistortion amount D is relatively small, the waveform distortion is notregarded as being detected, and the low-level waveform distortiondetection signal DT is outputted. On the other hand, if the waveformdistortion amount D is relatively large (e.g. if the waveform distortionratio is greater than or equal to approximately 30%), the waveformdistortion is regarded as being detected, and the high-level waveformdistortion detection signal DT is outputted.

As described above, by selectively correcting the waveform distortionwhen the waveform distortion is detected, it is possible to receive theaforementioned various effects while reducing the load of theinformation reproducing apparatus 1 f.

In addition, the signal level of the waveform distortion can becorrected to desired one of the signal level in the front edge portion,the signal level in the middle portion, and the signal level in the rearedge portion. Thus, it is possible to preferably correct the waveformdistortion in various shapes. Specifically, in the constructionexplained with reference to FIG. 7 to FIG. 9, the signal level of thewaveform distortion is corrected to the signal level in the front edgeportion. Thus, particularly, the waveform distortion in which the signallevel in the front edge portion is changed, as shown in FIG. 5( b) andFIG. 6( b), cannot be preferably corrected. According to the informationreproducing apparatus 1 f in the sixth modified example, however, thistype of waveform distortion can be preferably corrected.

Incidentally, the waveform distortion occurs generally due to thedispersion of the shape, length, and the like of the marks formed on therecording surface of the optical disc 100. Therefore, the waveformdistortion tends to occur in the recording type optical disc 100, suchas a DVD-R/RW, a DVD+R/RW, a DVD-RAM, and a BD-R/RE. However, even inthe read-only type optical disc 100, such as a DVD-ROM and a BD-ROM, thewaveform distortion occurs if the synchronization data formed of therelatively long mark is adjacent to each other in a tracking direction,as shown in FIG. 37. For the waveform distortion occurring in theread-only type optical disc 100, according to the informationreproducing apparatus 1 described above, the correction can bepreferably made, obviously.

The present invention is not limited to the aforementioned example, butvarious changes may be made, if desired, without departing from theessence or spirit of the invention which can be read from the claims andthe entire specification. An information reproducing apparatus andmethod, and a computer program, all of which involve such changes, arealso intended to be within the technical scope of the present invention.

1. An information reproducing apparatus comprising: an offset addingdevice for adding a first offset value which can be set to be variable,to a read signal read from a recording medium; a correcting device forcorrecting waveform distortion occurring in a read signal correspondingto at least a long mark, of the read signal to which the first offsetvalue is added by said offset adding device; an offset subtractingdevice for subtracting a second offset value which can be set to bevariable, from the read signal in which the waveform distortion iscorrected; and a waveform equalizing device for performing a waveformequalization process on the read signal in which the second offset valueis subtracted, wherein the first offset value is greater than the secondoffset value by a magnitude corresponding to a value set on the basis ofat least one of (i) an asymmetry value which indicates a shift amountbetween an amplitude center of a read signal obtained by reading recorddata with the shortest run length of the read signal and an amplitudecenter of a read signal which provides a maximum amplitude of a readsignal; (ii) an entire β value which indicates an average value of theamplitude center of the read signal; and (iii) a partial β value whichindicates deviation between the amplitude center of the read signalobtained by reading the record data with the shortest run length of theread signal and the amplitude center of the read signal obtained byreading the record data with the second shortest run length of the readsignal.
 2. The information reproducing apparatus according to claim 1,wherein the first offset value is same as the second offset value. 3.The information reproducing apparatus according to claim 1, wherein thefirst offset value is greater than the second offset value by amagnitude corresponding to a value obtained by multiplying the asymmetryvalue by an appearance probability, which does not consider the runlength, of the record data with the shortest run length with respect tothe record data included in the read signal.
 4. The informationreproducing apparatus according to claim 1, wherein the first offsetvalue is greater than the second offset value by a magnitudecorresponding to a value obtained by multiplying the entire β value byan appearance probability, which does not consider the run length, ofthe record data with the shortest run length with respect to the recorddata included in the read signal.
 5. The information reproducingapparatus according to claim 1, wherein the first offset value isgreater than the second offset value by a magnitude corresponding to avalue obtained by multiplying the partial β value by an appearanceprobability, which considers the run length, of the record data with theshortest run length with respect to the record data included in the readsignal.
 6. The information reproducing apparatus according to claim 1,wherein the first offset value is greater than the second offset valueby a magnitude corresponding to a value set on the basis of a positionalrelation between a reference level of the read signal and an amplitudecenter of a read signal obtained by reading record data with theshortest run length of the read signal.
 7. The information reproducingapparatus according to claim 6, wherein the first offset value isgreater than the second offset value by a magnitude corresponding to avalue indicating deviation between a reference level of the read signaland an amplitude center of a read signal obtained by reading record datawith the shortest run length of the read signal.
 8. The informationreproducing apparatus according to claim 1, wherein if reflectance of amark is smaller than reflectance of a space, at least one of the firstoffset value and the second offset value is less than a differencebetween a maximum value of a signal level of the long mark in which thewaveform distortion occurs and a reference level of the read signal. 9.The information reproducing apparatus according to claim 1, wherein ifreflectance of a mark is smaller than reflectance of a space, at leastone of the first offset value and the second offset value is a half of adifference between a maximum value of a signal level of the long mark inwhich the waveform distortion occurs and a reference level of the readsignal.
 10. The information reproducing apparatus according to claim 1,wherein if reflectance of a mark is greater than reflectance of a space,at least one of the first offset value and the second offset value isless than a difference between a minimum value of a signal level of thelong mark in which the waveform distortion occurs and a reference levelof the read signal.
 11. The information reproducing apparatus accordingto claim 1, wherein if reflectance of a mark is greater than reflectanceof a space, at least one of the first offset value and the second offsetvalue is a half of a difference between a minimum value of a signallevel of the long mark in which the waveform distortion occurs and areference level of the read signal.
 12. The information reproducingapparatus according to claim 1, wherein said waveform equalizing devicecomprises: an amplitude limiting device for limiting an amplitude levelof the read signal in which the waveform distortion is corrected, by apredetermined amplitude limit value, thereby obtaining an amplitudelimit signal; and a filtering device for performing a high-frequencyemphasis filtering process on the amplitude limit signal, therebyobtaining an equalization-corrected signal.
 13. The informationreproducing apparatus according to claim 1, wherein said offset addingdevice adds the first offset value and said offset subtracting devicesubtracts the second offset value (i) if an error correction of the readsignal cannot be performed, (ii) if an error rate of the read signal isgreater than or equal to a predetermined threshold value, or (iii) if aread signal corresponding to synchronization data cannot be read, thesynchronization data being used to read user data included in recorddata, the synchronization data being included in the record data. 14.The information reproducing apparatus according to claim 1, wherein saidcorrecting device corrects the waveform distortion (i) if an errorcorrection of the read signal cannot be performed, (ii) if an error rateof the read signal is greater than or equal to a predetermined thresholdvalue, or (iii) if a read signal corresponding to synchronization datacannot be read, the synchronization data being used to read user dataincluded in record data, the synchronization data being included in therecord data.
 15. The information reproducing apparatus according toclaim 1, wherein the long mark is a mark whose signal level is maximumamplitude.
 16. An information reproducing method comprising: an offsetadding process of adding a first offset value which can be set to bevariable, to a read signal read from a recording medium; a correctingprocess of correcting waveform distortion occurring in a read signalcorresponding to at least a long mark, of the read signal to which thefirst offset value is added by said offset adding process; an offsetsubtracting process of subtracting a second offset value which can beset to be variable, from the read signal in which the waveformdistortion is corrected; and a waveform equalizing process of performinga waveform equalization process on the read signal in which the secondoffset value is subtracted, wherein the first offset value is greaterthan the second offset value by a magnitude corresponding to a value seton the basis of at least one of (i) an asymmetry value which indicates ashift amount between an amplitude center of a read signal obtained byreading record data with the shortest run length of the read signal andan amplitude center of a read signal which provides a maximum amplitudeof a read signal; (ii) an entire β value which indicates an averagevalue of the amplitude center of the read signal; and (iii) a partial βvalue which indicates deviation between the amplitude center of the readsignal obtained by reading the record data with the shortest run lengthof the read signal and the amplitude center of the read signal obtainedby reading the record data with the second shortest run length of theread signal.
 17. A computer readable recording medium recording thereona computer program for reproduction control and for controlling acomputer provided in an information reproducing apparatus comprising: anoffset adding device for adding a first offset value which can be set tobe variable, to a read signal read from a recording medium; a correctingdevice for correcting waveform distortion occurring in a read signalcorresponding to at least a long mark, of the read signal to which thefirst offset value is added by said offset adding device; an offsetsubtracting device for subtracting a second offset value which can beset to be variable, from the read signal in which the waveformdistortion is corrected; and a waveform equalizing device for performinga waveform equalization process on the read signal in which the secondoffset value is subtracted, said computer program making the computerfunction as at least one portion of said offset adding device, saidcorrecting device, said offset subtracting device, and said waveformequalizing device, wherein the first offset value is greater than thesecond offset value by a magnitude corresponding to a value set on thebasis of at least one of (i) an asymmetry value which indicates a shiftamount between an amplitude center of a read signal obtained by readingrecord data with the shortest run length of the read signal and anamplitude center of a read signal which provides a maximum amplitude ofa read signal; (ii) an entire β value which indicates an average valueof the amplitude center of the read signal; and (iii) a partial β valuewhich indicates deviation between the amplitude center of the readsignal obtained by reading the record data with the shortest run lengthof the read signal and the amplitude center of the read signal obtainedby reading the record data with the second shortest run length of theread signal.